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Postharvest IPM Research Summaries: April 2000

Objective 1: Influence of preharvest factors on postharvest susceptibility to decay and insects.

Identification and etiology of visible quiescent infections of Monilinia fructicola and Botrytis cinerea in sweet cherry - J.E. Adaskaveg, Plant Pathology, UC Davis

    Brown rot and gray mold caused by the fungi Monilinia fructicola and Botrytis cinerea, respectively, can result in extensive losses in production of sweet cherry and other stone fruit crops worldwide. The often rapid increase in brown rot incidence of mature fruit regardless of protectant fungicide treatments or surface disinfestation have suggested the existence of symptom-less internal infections in stone fruit crops. Furthermore on sweet cherry, the common occurrence of necrotic flecks on green fruit or small reddish lesions on fruit of yellow-red varieties shortly after rains have also suggested visible quiescent (dormant) fruit infections. Both visible and non-visible infections that are established when environmental conditions or host physiology are conducive for fungal penetration, but not for active growth of the pathogen have been defined as quiescent infections. Isolation frequencies of fungi from necrotic flecks on green Bing fruit or from reddish halos on immature yellow-pink Rainier fruit over a four-year period indicated that M. fructicola was more common in 2 of the 4 years and was equal to B. cinerea in the other years. Using immature Bing cherry fruit, inoculation studies in the laboratory indicated that significantly more visible quiescent infections than active decays were produced in 6-, 9-, or 12-h than 18- or 24-h wetness periods after inoculation. The existence of non-visible quiescent infections of M. fructicola and B. cinerea in immature Bing and Rainier fruit collected 2 weeks before harvest was also substantiated using the postharvest paraquat technique. Furthermore, visible quiescent infections of M. fructicola were produced in the laboratory under defined, semi-conducive environments on immature fruit. This research further elucidates the disease cycles and epidemiology of brown rot and gray mold on sweet cherry fruit and provides insight and explanations to the difficulty in managing these diseases

Molecular characterization of Monilinia species and early detection of brown rot.
    The genetic diversity of Monilinia species was investigated using molecular techniques. Using RAPD analysis, multiple genotypes were common in M. fructicola but not in M. laxa indicating the genetic diversity of M. fructicola. Subsequently, we designed and evaluated primers from the ITS I region of ribosomal DNA and from random amplified polymorphic DNA (RAPD). A primer pair developed from RAPD amplified DNA for detection of M. fructicola and M. laxa and was very sensitive in detecting very early stages of fruit infection when no visual symptoms were evident. In laboratory studies, detection was possible in fruit that was incubated for as little as one day with a six-hour initial wetness period. Detection in naturally field-infected fruit was possible with moderate to high levels of infections (50-100% of fruit developed brown rot after 7-10 days of incubation). Currently, we are evaluating different fruit extraction methods. We also are evaluating new DNA technology for the capturing of rare DNA sequences out of a complex solution. Improved brown rot detection methods could facilitate postharvest management and marketing strategies of stone fruit crops

Blossom and preharvest fungicide treatments for brown rot control.
    With the loss of iprodione as a pre- and postharvest treatment on all stone fruits in 1999, we continued to evaluate, develop, and facilitate registration of new pre- and postharvest fungicides for management of blossom and fruit decays of fresh market stone fruit crops. Without the development of new classes of fungicides, the potential of resistant populations to develop to the solely registered DMI fungicides is greater than ever. New classes of fungicides including strobilurins, pyrimidinamines, hydroxyanilides, guanidinium compounds, and several numbered compounds were evaluated for management of brown rot on stone fruit. The DMI compounds (e.g., tebuconazole, propiconazole, myclobutanil, and fenbuconazole) and the pyrimidinamines (e.g., cyprodonil), and the strobilurins (e.g., azoxystrobin, trifloxystrobin) significantly reduced blossom blight on several stone fruit crops evaluated similar to results obtained with iprodione-oil treatments. In studies on the post-infection activity, tebuconazole, cyprodonil, and iprodione were highly effective when treatments were applied after inoculation. In preharvest studies (treatments 1 day, 7 + 1 day, 8 + 1 day, or 14 + 7 day PHI) using wound-inoculated fruit, the DMI fungicide tebuconazole was the most effective for brown rot management. Cyprodonil, fenhexamid, and selected numbered compounds also were very effective in reducing fruit brown rot. Gray mold of wound-inoculated fruit was most effectively controlled by cyprodonil. When preharvest-treated fruit were washed before inoculation, tebuconazole and cyprodonil were still very efficacious in reducing fruit decay, indicating that these fungicides persisted on the fruit tissue

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Factors influencing the susceptibility of lemons to fruit fly infestation. - David Obenland, Rodney Austin, and Dennis Margosan, USDA/ARS, Fresno
    The Citrus Research Board has requested that a study be done to determine if lemons are susceptible to fruit fly infestation. In conjunction to infestation work done by ARS-Hilo and ARS-Westlaco we are doing an accompanying project to find morphological and chemical factors that determine whether or not a lemon can be infested. Preliminary data has indicated that green lemons are non-susceptible but as they mature they can be infested. Fruit of different color classes, ranging from dark green to bronze will be evaluated over the entire season. The factors being examined include the quantity and distribution of various peel oils in the albedo, peel thickness and peel toughness. Data from the morphological and chemical evaluation of the peel will be compared and correlated with both Mediterranean and Mexican fruit fly infestation data.

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Objective 2: Benefits of postharvest temperature management or temperature treatments for control of decay and insects.

Low temperatures for control of stored-product insects. - Judy Johnson, USDA-ARS, Fresno

    We determined the exposures necessary for acceptable control of navel orangeworm and Indianmeal moth eggs with temperatures between 0-10°C. Temperatures in this range may take several weeks to kill larvae, pupae or adults, but high levels of egg mortality was achieved after 12, 10 and 8 days at 10, 5, and 0°C, respectively. We have proposed use of low temperature storage in combination with treatments that are more effective against older stages, or to protect recently disinfested product.

    In cooperation with Jim Thompson (Cooperative Extension, UC Davis) we have examined the potential of low temperature storage to protect postharvest prunes from insect infestation. To date, much of the work has been in developing baseline data on Indianmeal moth populations and an economic comparison of different cooling methods. An evaporative cooling system was found to be nearly as expensive as refrigeration, but provided insect control for only part of the year. The addition of cooling systems for insect control is not competitive with fumigants, but the cost may be acceptable if fumigants are not available, and has the advantage of improving product quality during storage.

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Heat treatment of peaches and nectarines for insect disinfestation. - David Obenland and Tanya Carroll, USDA/ARS, Fresno

    We have been evaluating the use of high temperature forced air for quarantine insect disinfestation of stone fruit. A drawback of heat treatment of stone fruit, however, is that the development of flesh mealiness is enhanced. Peaches and nectarines heated to a seed surface temperature of 47.2°C over a period of 4 h developed mealy flesh sooner and to a greater degree than non-heated fruit following cold storage for 1 to 3 weeks at 5°C. Polygalacturonase, an enzyme believed to be involved in the occurrence of this disorder, was reduced in activity following 3 to 4 h of heating and may be responsible for the enhanced mealiness. Pectinmethylesterase activity was unchanged in mealy tissue as compared to juicy. The amount of protein present was reduced by over 50% in the mealy fruit. Intermittent warming was ineffective in reducing the mealiness of heat-treated fruit. The effect of heat on this disorder must be considered if successful heat treatments for stone fruit are to be developed.

    Expansin is a protein involved in ripening that has properties that suggest a possible linkage to the development of mealiness. We are using Western blotting to monitor the amount of expansin protein present in peach tissue during the development of mealiness. Preliminary results suggest that the amount of expansin present in mealy tissue is substantially less than in juicy. Experiments in conjunction with Dr. Jocelyn Rose will be conducted to evaluate expansin gene expression in mealy fruit.

    As a means to evaluate changes in protein expression as a result of stresses, such as that due to heat treatment, we have developed the methodology to perform two-dimensional polyacrylamide gel electrophoresis on stone fruit tissue. We will utilize this technology to help us better understand the nature of stress-induced disorders so that means to maintain fruit quality during and following treatment can be developed.

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Objective 3: Influence of atmospheric modification on decay and insect control (CA, MA, CO, acetaldehyde, ozone).

Natural volatiles from sagebrush for insect control. - Judy Johnson, USDA-ARS, Fresno

    In cooperation with Florence Dunkel (Montana State University), we have conducted preliminary tests on the control efficacy of natural volatiles from sagebrush against Indianmeal moth and navel orangeworm. Navel orangeworm eggs and pupae proved to be quite susceptible to the volatiles.

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Quarantine control of insects on table grapes. - Elizabeth Mitcham, Pomology, UC Davis

    We are continuing research to develop an effective treatment for control of omnivorous leafrollers, western flower thrips, Pacific spider mites and grape mealybug on table grapes. A 13 day treatment at 2C with 45% CO2 controls all lifestages of these pests, except grape mealybug. Diapausing mites have not been tested, but will be tested soon. The effects of prior cold storage on the response of insects and mites is under study. We are exploring other treatment options for grape mealybug. A combination treatment of 10% CO2 plus 0.1% SO2 was effective against grape mealybug crawlers, but not eggs or adults. Combination with higher levels of SO2 will be tested. In addition, fumigation with GRAS volatile compounds will be explored for control of grape mealybug. When a promising treatment is identified, grape fruit response will be tested.

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Acetaldehyde for control of insects on chrysanthemum cuttings and strawberry fruit. - Elizabeth Mitcham, Pomology, UC Davis

     Fumigation with acetaldehyde (0.5 to 3%) is being tested alone and in combination with CO2 for control of two-spotted spider mites, western flower thrips and green peach aphids. Control of insects and mites appears promising; however, the absorption of volatile compounds by plant materials must be considered in treatment efficacy and product tolerance. We are in the process of developing response curves for mortality in the presence and absence of chrysanthemums or strawberry fruit. Other volatile compounds such as methyl formate and acetic acid will also be tested.

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Mode of action of low O2 and high CO2 atmospheres. - Elizabeth Mitcham, Pomology, UC Davis

     We recently completed a study of the response of omnivorous leafroller pupae to low O2 and high CO2 atmospheres using microcalorimetry and respiration rates. Our findings support results with other non-quarantine insect species and fungi. Low O2 atmospheres effect insects and pathogens by reducing respiration rates and lowering energy charge. In addition, the induction of anaerobic metabolism produces toxic products that result in cell damage. Elevated CO2 has a similar effect on cells; however, CO2 has an additive effect of increasing membrane permeability and ion leakage. This results in a faster reduction in energy charge under CO2 atmospheres due to a less efficient production of ATP. Carbon dioxide reduces respiration by inhibiting succinate dehydrogenase, while low O2 effects oxidative phosphorylation. With a similar effect on the metabolic rate, CO2 resulted in higher mortality of omnivorous leafroller pupae. It remains to be tested if our results can be generally applied to other insect species; however, there do appear to be a number of similarities in the effects of low O2 and elevated CO2 on the cell.

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Mortality of spores of Botrytis cinerea, Monilinia fructicola, Penicillium digitatum, and Rhizopus stolonifer after exposure to ozone under humid conditions. - D. A. Margosan and J. L. Smilanick, USDA-ARS, Fresno

    Spores of Botrytis cinerea, Monilinia fructicola, Penicillium digitatum, and Rhizopus stolonifer from two-week old fungal cultures were transferred to sterile glass coverslips and treated with ultraviolet light-generated ozone at a constant concentration at 130 ppm at 15°C and +95% rh for 10, 20, 40, 80, and 160 minutes. After exposure to ozone, the spores were incubated on potato dextrose agar for 18 hours at 23-25°C. Germination was determined by examination at 25X. The test was repeated twice. All of the spores were killed after 80 to 160 min exposure, except R. stolonifer, which was more tolerant than the other fungi. The concentration times time (C x T) products for 50% mortality for B. cinerea, M. fructicola, P. digitatum, and R. stolonifer were 26.3, 26.9, 59.4, and 66.0 respectively; for 95% mortality (LD-95) the C x T products were 108.6, 57.1, 110.3, and 231.1 respectively. Ozone toxicity was also assessed at low RH (ca. 30%) and was about five-fold less toxic. Ozone toxicity was assessed also at 5 and 25°C. Compared to results at 15°C at low RH, ozone was less toxic at 5°C and more toxic at 25°C. These values can be used to estimate dosages of ozone needed for disinfection or sanitation purposes.

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Control of spores of postharvest fungal pathogens of produce with ozone. - J. L. Smilanick, D. A. Margosan, F. Mlikota, USDA-ARS, Fresno; J. T. C. Yuan, American Air Liquide, Countryside, IL; and B. C. Hampson, Department of Food Science and Nutrition, California State University, San Luis Obispo.

    Water used in processing produce can carry spores of pathogens that reduce shelf-life. Ozone presumably can sanitize process water, but doses required are needed. Our objectives were to determine O3 doses that kill spores, and the influence of pH and O3 concentration on O3 potency.

    O3 was made with a corona discharge unit and dissolved in water in a contactor attached to a 1000-L tank. O3 concentration was monitored with an O3 electrode and did not change during tests. Ozonated water pH was 6.4 in spore mortality tests, except when adjusted to pH 5 or 8 with H2SO4 or KOH in water buffered with 2 mM Na2HPO4. Spores of Penicillium digitatum, Penicillium italicum, Penicillium expansum, Monilinia fructicola, Rhizopus stolonifier, Botrytis cinerea, and Geotrichum citri-aurantii from 2-wk old colonies on potato dextrose agar (PDA) were collected in 2 ml water, centrifuged, and resuspended in water at 106 spores/ ml. A 0.2 ml aliquot was placed on a 3-µ pore size filter, low-pressure vacuum dried, then 1 - 4 µg O3 /ml flowed through the spores at 1.6 ml/sec. To end exposure, 3 ml of 1000 µg/ml calcium thiosulfate was added, then 5 ml water. The filter was dried, removed, inverted, and touched to PDA, where the spores were deposited. Germination was recorded after 14 - 24 hr at 20°C.

    Two min in 1.5 µg O3 /ml killed 95-100% of spores. None survived 3 min of contact, and this dosage, a C x T product of 4.5 µg O3 /ml x min., could be used to treat packinghouse process water to control these fungi. Spores died similarly at pH 5 and 8. LD50 times of P. digitatum spores in 1, 2, and 4 µg O3 /ml were 21.6, 10.4, and 6.4 sec, respectively. LD50 CTs were 0.36, 0.35, and 0.42 µg/ml x min, respectively.

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Mortality of Botrytis cinerea, Monilinia fructicola, Penicillium digitatum, and Rhizopus stolonifer spores after exposure to ozone gas. - D. A. Margosan, and J. L. Smilanick, USDA/ARS, Fresno

    Spores of Botrytis cinerea, Monilinia fructicola, Penicillium digitatum, and Rhizopus stolonifer were exposed to ozone at a concentration at 120 to 200 ppm at low (35 ± 3% RH) or high (= 95% RH) humidity at 5, 15, or 25°C for 5 min to 72 hours (until mortality equal to or greater than 95% occurred). After exposure, spore viability was determined by incubation on potato dextrose agar for 18 hours at 23-25°C and germination was determined by examination at 250X. Dosages that killed 50% (LD50) and 99% (LD99) of the spores were estimated and expressed as the product of ozone concentration (1 or 100 ppm) multiplied by time (hours). Each test was repeated three times.

    R. stolonifer was consistently more tolerant to ozone than the other fungi (Figure 1). Ozone was about six times more toxic at high relative humidity than at low humidity. For example, at 15°C, the average LD50 for all the fungi at high humidity was 96.5 ppm*hr, while at low relative humidity it was 573 ppm*hr. At low humidity, ozone toxicity increased with temperature; it was approximately 5 times more toxic at 25°C than 5°C. At high humidity, ozone was most toxic at 25°C, followed by 5°C, and least toxic at 15°C. At low humidity, ozone was most toxic at 25°C, followed by 15°C, and least toxic at 5°C.

    Our results estimate the doses of ozone gas needed to kill spores of these fungi. The doses needed are very high and greatly exceed doses in which workers can operate safely (OSHA limits are 0.1 ppm in 8 hr day and 0.3 ppm for 15 min). Furthermore, many fresh commodities would be injured after exposure to sporocidal ozone doses, although this has been incompletely evaluated. Some durable commodities (nuts, small and large grains, dried fruits) would probably tolerate these doses without visible injury, but this also needs further evaluation.

    Although not included in this study, inhibition of spore production by ozone occurs at far lower concentrations (ca. 0.3 ppm) than those that kill spores. Inhibition of spore production can be valuable for sanitation purposes for products in storage.

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Evaluation of the effect of ozone exposure on decay development and fruit physiological behavior. - Lluís Palou and Carlos H. Crisosto, Dept. of Pomology, UC Davis; Joseph L. Smilanick, USDA-ARS, Fresno; James E. Adaskaveg, Dept. of Plant Pathology, UC Riverside; and Juan P. Zoffoli, Dept. de Fruticultura, Universidad Católica de Chile

The influence of ozone on decay incidence and development of several fungi was evaluated on artificially inoculated peaches, oranges, and table grapes. Ozone at 0.3 ppm (OSHA short-term exposure) did not affect decay incidence and severity on 'Elegant Lady' peaches inoculated with Botrytis cinerea, Penicillium expansum and Mucor piriformis after 4 wk storage at 5°C. The incidence of brown rot, caused by Monilinia fructicola was delayed 1 wk and brown rot severity was significantly lower during the storage period. Ozone exposure at 5°C altered normal mycelium growth and inhibited sporulation. However, normal mycelium growth resumed after fruit were removed from ozone. The incidence of citrus green and blue molds on inoculated 'Valencia' oranges was significantly delayed by 1 wk under 0.3 ppm ozone at 5°C. Green and blue molds severity was slightly lower under ozone. Ozone exposure prevented the sporulation of both pathogens. Ozone at 0.3 ppm eliminated gray mold (B. cinerea) nest formation on exposed 'Thompson Seedless' table grapes stored for 7 wk at 5°C. Ozone exposure (0.3 ppm for 3 wk) did not affect peach fruit respiration. However, after 4 wk exposure to 0.3 ppm ozone peaches became more susceptible to water loss. Work to understand the role of ozone in our industry is still in progress.

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Objective 4: Methods to improve postharvest handling or sanitation procedures to reduce susceptibility to decay or insects.

Plastic bin liners to prevent postharvest infestation of prunes. - Judy Johnson, USDA-ARS, Fresno

    In cooperation with Jim Thompson (Cooperative Extension, UC Davis), we are investigating the utility of plastic bin liners to prevent infestation of postharvest prunes by newly hatched Indianmeal moth larvae. Our studies indicate that neonate Indianmeal moth larvae are incapable of penetrating plastic bags, but are adept at finding and entering through minute holes in the bags, particularly along the seals. The efficacy of the bags under high Indianmeal moth population pressure is currently being tested

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Control of postharvest decay of citrus fruit with calcium polysulfide. - Joseph L. Smilanick, USDA-ARS, Fresno, and David Sorenson, Sunkist Growers, Technical Services, Lindsay, CA

    Incidence of green mold of citrus, caused by Penicillium digitatum, was reduced 80% or more by the immersion of lemons or oranges for 1 to 4 min in warm (40.6 to 43.3°C) "liquid lime-sulfur" solution (LLS) that contained 0.75% (wt/vol) calcium polysulfide. The incidence of sour rot, caused by Geotrichum citri-aurantii, was reduced 35 to 70% by this treatment. The treatment was similar in effectiveness to other treatments employed to control postharvest decay. LLS was more effective on lemons than oranges, and on green compared to yellow lemons. LLS did not stop sporulation from lesions that did develop, a benefit obtained with some fungicides. Residues on LLS-treated fruit were determined by distillation of the fruit at low pH followed by titration or HPLC analysis of the distillate. The sulfide content of oranges, lemons, and grapefruit after LLS treatment was 31.9, 33.1, and 36.3 µg/g, respectively. Rigorous cleaning of fruit with water applied at high pressure after LLS treatment slightly improved LLS efficacy; conversely, similar cleaning reduces the efficacy of sodium carbonate or borax-boric acid solutions now in use. This facilitates LLS adoption because it can be inserted into existing processing lines either before or after the fruit are cleaned. The risk of injury to fruit by LLS was low. Fruit of one lemon and five navel orange cultivars were not visibly injured after LLS treatment for 3 min at 40.6°C followed by storage at 10°C for 7 wk. After LLS treatment at 48.9°C, a temperature 5°C higher than needed for effective LLS use, only Lisbon lemons and Bonanza navel oranges were slightly injured. LLS solution needs periodic replenishment during use. Sulfide concentration in LLS solution declined at a rate of about 7% every 24 h, this rate was similar between 25 and 65°C, and it was accompanied by the appearance of resistant deposits on the equipment. Additional losses would occur when some LLS solution is carried on fruit out of the tank. Although hydrogen sulfide in the air above LLS solution in pilot tests was less than 1 ppm and below the worker safety threshold of 10 ppm, LLS solution has an odor of hydrogen sulfide that can be a nuisance to workers. The disposal of used LLS solutions is more readily accomplished than other tank treatments whose disposal can be difficult because they contain synthetic fungicides, are caustic, or have a high salt concentration. Because LLS improves water penetration in soils and is commonly used for this purpose, in many locations it can be disposed of by application to agricultural soils. LLS was approved for postharvest citrus fruit by the USEPA and CalDPR in November, 1998.

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Postharvest control of table grape gray mold with carbonate and bicarbonate salts and disinfectants. - F. Mlikota Gabler, Institute for Adriatic Crops, Croatia, and J. L. Smilanick, USDA-ARS, Fresno

    The shelf-life of table grapes is often limited by postharvest gray mold. Control of gray mold is particularly difficult in mixed-fruit packages containing detached berries that cannot be effectively fumigated with sulfur dioxide. The control of gray mold by treatment with carbonate and bicarbonate salt solutions, alone or with chlorine, ozone, or ethanol, was evaluated. Sodium carbonate (SC), potassium carbonate (PC), sodium bicarbonate (SBC), potassium bicarbonate (PBC), and ammonium bicarbonate (ABC) were tested without control of pH for their toxicity to spores of Botrytis cinerea in vitro, and the EC-95 concentrations were 16, 17, 36, 58, and 163 mM, respectively. When bicarbonate solutions were tested at pH 7.2 (±0.2), the ED50 concentrations of ABC, SBC, and PBC were 26, 46, and 48 mM, respectively. The LD50 and LD95 mortality of spores of B. cinerea occurred after 21.3 and 35.6 seconds exposure to 1.5 µg/ml of ozone in water, respectively. In practical tests to control gray mold on grapes, among the bicarbonates, each applied at 500 mM, ABC was significantly more effective than SBC and PBC. It was also superior to PC (100 mM) and chlorine (200 µg/ml), and equal in effectiveness to SC (100 mM) and ethanol (70% wt/vol). The addition of 200 µg/ml chlorine to the bicarbonate salts significantly decreased gray mold incidence. Ozone in water significantly controlled gray mold, although its efficacy was irregular and very dependent on grape condition. Among all the treatments, berry condition was an important factor; there was significant decrease in control when wounded berries were treated compared to unwounded berries. The quality of grapes after treatment with ABC, SBC, ethanol and chlorine was acceptable, while severe injuries, mostly brown spots on berries, occurred after SC, PC, and PBC treatments.

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New fungicides for postharvest decay management. - James E. Adaskaveg, Plant Pathology, UC Davis

    One of our research objectives is to find alternative postharvest fungicides to previously registered materials. Of the many new fungicides being developed and registered, the most exciting ones are the "Reduced Risk" pesticides. This classification by the United States Environmental Protection Agency is a relative term applied to a pesticide as compared to other currently registered pesticides in a crop group. Information on these pesticides is available on the worldwide web at http://www.epa.gov/oppfead/fqpa/1205mtng/redcrisk.html. A pesticide is considered of reduced risk when it broadens the adoption of integrated pest management practices or reduces the exposure risk to humans, the potential toxicity to non-target organisms, or the contamination of environmental resources. Among the seven fungicides that are currently designated as "reduced risk", representatives of the classes strobilurins (azoxystrobin and trifloxystrobin), phenylpyrroles (fludioxonil), and hydroxyanilides (fenhexamid) are being developed for postharvest use on selected fruit crops including citrus, stone and pome fruits, and kiwifruits in the United States. Of these reduced risk materials, the strobilurins and the phenylpyrroles are compounds that were synthesized based on the structure of naturally occurring products. Based on our efficacy results, fludioxonil is being registered on stone fruits for brown rot, gray mold, Rhizopus rot, and Gilbertella decay; azoxystrobin is being developed on citrus against Penicillium decays; whereas fenhexamid is being registered on stone fruit for brown rot and gray mold and on pome fruit and kiwifruit for gray mold management. All of the fungicides are highly active against target organisms. This has resulted in improved decay control using lower rates (and lower projected tolerances) than previously registered fungicides.

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Objective 5: Influence of energy wave inputs such as irradiation, UV, or microwave on postharvest decay and insect infestations.

Pulsed ultraviolet light and radio frequency treatments. - Manuel C. Lagunas-Solar, Crocker Nuclear Laboratory, UC Davis

    The Chemistry & Agriculture Program at the Crocker Nuclear Laboratory, in collaboration with several departments at UC Davis, is investigating the applications of pulsed ultraviolet light (PUV) and of selected radio frequency bands (RF) to affect the reproduction mechanisms of pathogenic organisms in several media, including fresh (fruits & vegetables), processed foods, waste materials, and many other non food materials such as soils and wood. PUV has been shown to affect microbial flora on the surface of fresh fruits and control pathogenic bacteria (> 7 log reduction) and viruses (>7 log reduction) in several transparent or partially transparent liquid fluids such as drinking water, fruit juices, milk, and plasma (US patent # 5,364,645, other patents pending). In addition, PUV is effective for surface insect & mites disinfestation applications (US Patent # 5,607,711). The commercial use of PUV is being investigated for several processes. RF techniques have been shown to be effective for disinfection and disinfestation applications on similar materials as PUV. However, contrary to the surface effects of PUV or its limitations in depth of penetration in liquid fluids, RF interacts homogeneously in the bulk of a material. Therefore, in addition to fresh fruits, disinfection, disinfestation, and desirable biological effects in several agricultural commodities, in waste materials, animal and human diet products, soils, and wood products, have been shown in a laboratory scale. Several commercial applications are being evaluated.

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Radio frequency for insect and pathogen control. - Elizabeth Mitcham, Pomology, UC Davis, and Juming Tang, Washington State University

    Radiofrequency for insect and pathogen control. Electromagnetic radiation at frequencies lower than microwave are under study as a rapid method of heating fresh and dried commodities for insect and possibly pathogen control. Radio frequency (RF) preferentially heats insects over commodities and the differential in the rate of heating is even greater for dried commodities such as walnuts. RF has the benefit of heating the product from the inside out. Products can be heated within a few minutes, and heating is uniform if the product is suspended in a water solution with a small amount of salt that then also heats. Commercial units are presently used for disinfection of baked products. This heating process is so rapid that it is feasible as an on-line system. A project with walnuts will begin as soon as funding is confirmed. Research with sweet cherry is underway.

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Objective 6: Use of natural enemies for biological control and increased understanding of basic biology of postharvest insect pests and decay organisms.

Parasitoids of stored-product insects. - Judy Johnson, USDA-ARS, Horticultural Crops Research Laboratory

    We have determined that Habrobacon hebetor, an external parasitoid of pyralid moth larvae, is capable of paralyzing and killing diapausing Indianmeal moths during the winter. We are investigating the release of H. hebetor during the winter as a means of reducing Indianmeal moth populations in dried fruit and nut storage. Initial results with this method on almonds are encouraging.

    We have also looked at the survival of Venturia canescens, an internal parasitoid of pyralid larvae, in diapausing hosts subjected to below freezing temperatures. V. canescens is capable of surviving exposure to sub-freezing temperatures, provided the host survives.

    We have found an undescribed chalcid species parasitizing pyralid pupae. The life history of the new parasitoid is being investigated, and its potential as a biological control agent will be addressed.

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Indianmeal moth granulosis virus. - Patrick V. Vail, J. Steven Tebbets, and Darlene F. Hoffmann, USDA-ARS, Fresno, CA

    The Indianmeal moth (IMM) granulosis virus (GV) was first isolated and characterized in 1968 by Arnott and Smith. As a group, the Baculoviridae are considered to be safe because extensive testing has demonstrated their safety. Twelve have been registered world-wide. In the late 1960s, interest increased in the use of microbial agents to control stored product pests in durable commodities. Studies by Hunter et al. (1973) demonstrated efficacy against IMM infestations in raisins, almonds and walnuts. Cowan et al. (1986) developed a production/formulation method for IMM-GV which was later patented (Vail 1991). Johnson et al. (1998) incorporated the GV in tests combined with controlled atmospheres for initial disinfestation of IMM populations in unshelled walnuts followed by treatment with the GV for long term storage. The IMM-GV provided excellent protection of walnuts even with severe IMM pressure for a period of 4 months. Similar tests were conducted with almonds.

    After the tests described above were completed, we determined the persistence/efficacy of GV on treated nuts through a period of 2 years. Immediately after the initial 4 month test, treated and control walnuts and almonds were frozen to kill any IMM remaining in the commodities. Thereafter walnuts or almonds were removed and placed in sealed storage at 80°F. Samples were removed bimonthly, infested with approximately 1,000 IMM eggs, and incubated at 80°F for 6 weeks. The tests were replicated twice in time. Following incubation the nuts were observed for numbers of IMM adults and damage (pin hole, moderate, and severe).

    Results of these studies showed that IMM-GV would persist for at least 2 years. As time passed, the titre of the virus gradually was reduced as was evidenced by increase in damage in time. However, moth emergence was reduced by more than 90% through the test period with both commodities. Damage to almonds exceeded that of walnuts probably due to easier access of larvae to the kernels by way of cracked and thinner shells. Infestation rates used in these tests are unreasonably high and bias the data towards more damage than would likely occur commercially. Our grading procedures are also likely to be much more rigid than would be used commercially. The virus is now proceeding through registration with the United States Environmental Protection Agency.

References:

  • Arnott, H. J. and K. M. Smith. 1968. An ultrastructural study of the development of a granulosis virus in the cells of the moth, Plodia interpunctella (Hübner). J. Ultrastruct. Res. 21: 251-268.
  • Cowan, D. K., P. V. Vail, M. L. Kok-Yokomi and F. E. Schreiber. 1986. Formulation of a granulosis virus of Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae): Efficacy, persistence, and influence on oviposition and larval survival. J. Econ. Entomol. 79: 1085-1090.
  • Hunter, D. K., S. S. Collier and D. F. Hoffmann. 1973. Effectiveness of a granulosis virus of the Indian meal moth as a protectant for stored inshell nuts: preliminary observations. J. Invertebr. Pathol. 22: 481.
  • Johnson, J. A., P. V. Vail, E. L. Soderstrom, C. E. Curtis, D. G. Brandl, J. S. Tebbets, and K. A. Valero. 1998. Integration of nonchemical, postharvest treatments for control of navel orangeworm (Lepidoptera: Pyralidae) and Indianmeal moth (Lepidoptera: Pyralidae) in walnuts. J. Econ. Entomol. 91: 1437-1444.
  • Vail, P. V. 1991. Novel virus composition to protect agricultural commodities from insects. U.S. Patent No. 07/212,641.

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Biological control, heat and CA for control of Botrytis rot. - Elizabeth Mitcham, Pomology, UC Davis

    Graduate student, Annette Wszelaki, is conducting a project to test the benefits of combined use of heat therapy, biological control and controlled atmosphere treatments for control of Botrytis cinerea using strawberry fruit as a model system. Both newly isolated microflora from California strawberry fruit and previously confirmed biological control agents against B. cinerea are being tested. Efficacy of single treatments have been tested. Combination tests will be initiated soon.

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