Fruit Pest News

August 20, 2001

An online newsletter whose goal is to update Extension agents and growers of commercial tree fruit and small fruit crops

on diseases and insects in Tennessee.

Fruit Pest News will be produced every two weeks until the end of the 2001 growing season. The weekly schedule will resume next spring.

Text appearing in blue or red can be cllicked to link to other web sites. Be aware that much of the linked information is produced in other states and may not be applicable to Tennessee.

In This Issue:

    1. Late-Season Apple Insects

    2. Post-Harvest Treatment for Apples

 1. LATE-SEASON APPLE INSECTS

Don't forget about insects in the later part of the summer. Lots of time and effort has gone into producing high quality fruit and we want to make sure it stays that way through harvest and storage. Oriental fruit moth (OFM) has become a more important pest on apples in recent years. Particular attention should be paid to this pest in August and September for the 3rd and 4th generations. Continue to monitor your OFM pheromone traps. Since insecticides should give 2 weeks control do not be concerned if trap catches remain high one week after spraying. OFM can be controlled with mating disruption if insecticides are used against the first generation and Isomate-M100 dispensers are applied at the 2nd and 3rd cover.

Codling moth should also be monitored using pheromone traps. Inspect fruit for codling moth damage (messy excrement coming out of holes in the fruit). Treat if damage is found or if trap catches for the third generation exceed 5 moths/trap/week which should be1,900 DD after biofix.

It is recommended to treat for white apple leafhopper or potato leafhopper in late July to early August if one nymph or more are found per leaf. Treating at this time (it may not be too late if mainly nymphs are found) will eliminate the need to treat for the adults immediately before harvest.

European red mites and twospotted spider mites can reproduce rapidly in this hot, dry weather. The effectiveness of miticides is rated from + = poor control to +++++ = excellent control in the 2000 Integrated Orchard Management Guide for Commercial Apples in the Southeast. The most effective miticide is probably Pyramite 60WP (+++++) while Kelthane 50WP and Vendex 50W are rated (++++). Carzol 92SP, Vydate 2L, and summer oil are all rated (++) while wettable sulfur is rated (+). (FH)

2. POSTHARVEST TREATMENTS FOR APPLES: The majority of postharvest decays of apples are caused by Penicillium expansum (the cause of blue mold) or Botrytis cinerea (the cause of gray mold). These fungal pathogens usually infect fruit at wounds, but they can also invade the fruit through the stem or at bruises. Postharvest diseases traditionally have been controlled by applying postharvest fungicide drenches to apples before they are moved into refrigerated or controlled atmosphere (CA) storage.

Varieties such as Delicious and Cortland are highly susceptible to the physiological disorder known as storage scald. These varieties must be treated with diphenylamine (DPA) to prevent scald during CA storage. A fungicide is always included with the DPA treatment to control infections that would otherwise develop as a result of inoculum that accumulates in the postharvest treatment solutions. Some varieties do not develop scald and therefore can be moved into storage without any postharvest treatment.

Of the two major postharvest pathogens, blue mold can be the more difficult to control. When postharvest treatments fail to control Penicillium, two explanations are possible. The first, and the one with the easiest solution, is that fungicide concentrations in the postharvest drench may be too low. This can occur if the fungicide is not replenished according to label instructions as apples are processed through the drencher. It also occurs when the fungicide settles to the bottom of drencher holding tanks due to the lack of adequate agitation systems. Mertect 340F (thiabendazole, or TBZ) is the most commonly used postharvest fungicide. This fungicide appears to settle out of suspensions much more quickly and completely than did Topsin-M or Benlate, fungicides that were used in the 1970s and 1980s. As a result, Mertect 340F may settle to the bottom of the holding tank and remain there in the absence of good agitation. This problem can be rectified by installing an agitation system in the bottom of the holding tank.

A second reason that postharvest fungicide treatments might fail involves resistance to the fungicides. Certain strains of Penicillium and Botrytis are resistant to all the benzimidazole fungicides, including TBZ. In New York, in the mid-1980s, researchers discovered that most of the fungicide-resistant strains of Penicillium and Botrytis were unusually sensitive to DPA. Thus, the combination of DPA plus a benzimidazole fungicide continued to provide good control of Penicillium and Botrytis through the late 1980s. Even in the mid-1980s, however, about 2% of the P. expansum strains recovered from apple storages were resistant to both DPA and TBZ. It appears that these strains have gradually increased in importance and are at least partially responsible for declining effectiveness of postharvest treatments.

There are no easy solutions for controlling resistant strains of Penicillium. Sanitation measures that reduce the amount of inoculum harbored on bins and storage walls may help to reduce the incidence of decay, but the benefits of sanitation have not been studied or documented in apple storages.

One approach for controlling TBZ-resistant strains of Penicillium would be to add captan to the postharvest treatment solution. Captan is registered for postharvest use on apples and can be combined with DPA and TBZ in postharvest treatments. Researchers in Israel have reported that captan is reasonably effective against Penicillium when used at the full label rate of 2.5 lb Captan 50W per 100 gallons. In the US, where captan has usually been tested at lower rates and in combinations with one of the benzimidazole fungicides, captan has provided no benefits compared with using the benzimidazole fungicide alone.

Several biocontrol fungicides have been registered for postharvest use on apples and may eventually prove useful for controlling strains of Penicillium that are resistant to TBZ. Biocontrol fungicides are formulations of bacteria or yeast - living organisms that actually grow on the fruit after they are applied. Biocontrols do not act by killing pathogen spores or inhibiting spore germination. Instead, they stop decays by colonizing the wounds on apple fruit where decays are usually initiated. The biocontrol organisms apparently utilize all of the available nutrients in the wounds, leaving nothing to support initial growth of the decay fungi. The decay fungi utilize the apple juice and damaged cells in wounds as a source of nutrients for initial growth of spores. When this "start-up fuel" is consumed by the biocontrol fungi, the pathogens are left without the nutrients needed to initiate growth. This mode of action for biocontrols dictates that biocontrol fungicides will have good protectant activity but virtually no eradicant activity. If Botrytis or Penicillium become established before the biocontrol is present, then the decay fungi will usually predominate and continue to invade and eventually decay the fruit.

Decco I-182 is a formulation of the yeast Candida oleophila that was formerly marketed as 'Aspire'. The limited test data available for apples suggests that this product will provide only modest improvements in decay control when applied in combinations with TBZ. Storage operators may wish to experiment with Decco I-182 this fall, but we do not yet have enough information to recommend this expensive product. Tests conducted in Virginia in 1996 showed that Aspire by itself at 40 and 80 oz./100 gallons of water failed to control blue mold.

Biocontrol fungicides, such as Decco I-182 or Biosave, can be ordered through agrichemical suppliers but will not be warehoused with other agrichemicals. Because these products contain a living organism, they must be kept refrigerated until use. When the product is ordered, it will be shipped directly from the manufacturing plant to the purchasers in insulated containers. Drench solutions that include Decco I-182 will have a characteristic yeasty odor that some individuals may find offensive, but the odor apparently is not retained on treated fruit.

For fruit that will NOT be given postharvest treatments, storage operators may wish to employ some of the following precautions to limit the potential for postharvest decay problems:

• One or two applications of Benlate-Captan or Benlate-Ziram should be applied in late summer, with the last application sometime between 15 to 21 days before harvest. Benlate will provide better residual activity than Topsin M. Note the 14-day preharvest interval for Ziram.

• Inoculum levels are likely to be higher on fruit that are hanging less than 18 inches from the ground at harvest. Postharvest pathogens survive in soil beneath trees and are likely to be rain-splashed onto low-hanging fruit. Fruit from orchards with a lot of low-hanging fruit are at greater risk for postharvest decays and may benefit from a postharvest treatment.

• Inoculum for postharvest decays can accumulate and survive on wooden apple bins. If the option is available, use newer bins or plastic bins for fruit that will not be drenched.

The following guidelines should be followed whenever postharvest treatments are applied to apples of any variety:

• Always apply DPA and TBZ together. Neither product should be applied alone in a postharvest drench. Some of the postharvest pathogens are resistant to TBZ, but the TBZ-resistant isolates are usually controlled by DPA. Thus, both products are required to control the full spectrum of pathogens. As noted earlier, DPA should not be applied alone because the DPA drench will collect spores and will increase the incidence of decay in fruit if a fungicide is not included in the drench.

• Never mix any chlorine products with DPA/fungicide treatments. Chlorine disinfectants may react with DPA and Mertect.

• Cool fruit as rapidly as possible after treatment. The activity of DPA against TBZ-resistant pathogens is very limited at temperatures above 40 F. If apples are cooled slowly, TBZ-resistant Penicillium can initiate decays before the antifungal activity of DPA "kicks in".

• Keep drench solutions agitated. With Mertect 340F, inadequate agitation in the drench tank will prove disastrous because all of the active ingredient will settle to the bottom of the tank. The best system for agitating drench tanks involves the use of a high-volume pump to recirculate water through PVC "jets" that direct water flow across the bottom of the reservoir tank and create turbulence within the tank. An agitation system should be considered inadequate if it is not capable of resuspending virtually all of the sediment from the bottom of the tank when the system is activated after an overnight shut-down.

• Keep drench solutions clean. Soil introduced into the postharvest treatment tanks carries decay inoculum and makes it more difficult to keep postharvest chemicals in suspension. A pre-wash with a high-volume stream of non-recycling water may be needed to remove soil from bins or equipment before they enter the postharvest drencher. Empty and clean tanks at least as frequently as is required on the DPA labels.

• Keep drench solutions properly recharged. The drench solutions should be regularly recharged according to instructions included on the postharvest labels of the products being used.

Captan is also registered for postharvest use and can be added to the DPA/Mertect solution. However, Captan has shown only marginal activity against postharvest decays in tests in New York and Virginia. Proper disposal of dip tank water is required by federal law. Mertect may be disposed of on-site, although no specifics (other than to prevent contamination of water, food, or feed) are provided on the label. Captan may be disposed of by spraying the dip tank solution onto either apples or cherries, or on to registered turf or ornamental sites (observing the restrictions such as maximum amounts per application and season).

This article was adapted by Alan Biggs, West Virginia University, from material prepared by D. A. Rosenberger, Cornell University, Hudson Valley Laboratory, Highland, N.Y.

The Fruit Pest News URL is: http://web.utk.edu/~extepp/fpn/fpn.htm

Contacts:

Steve Bost, Professor and Extension Plant Pathologist

scbost@utk.edu

Frank Hale, Associate Professor and Extension Entomologist

fahale@ext1.ag.utk.edu

Both authors available at:

615-832-6802

fax 615-781-2568

Plant and Pest Diagnostic Center

5201 Marchant Drive

Nashville, TN 37211