Fruit Pest News

Volume 7, No. 20 August 24, 2006

An online newsletter whose goal is to provide all interested persons with timely information on diseases and insects of commercial fruit and vegetable crops in Tennessee.

Text appearing in blue or red can be clicked 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. Plum Pox Virus Found in Michigan 2. Strawberry: Fungicide Use on Plasticulture Strawberries in the Fall 3. Apple Fruit Rot Diseases 4. Grape Downy Mildew 5. Cucurbits: Using Sulfur on Cucurbit Crops

1.Plum Pox Virus Found in Michigan

WASHINGTON, Aug. 11, 2006--The U.S. Department of Agriculture’s National Plant Germplasm and Biotechnology Laboratory in Beltsville, Md., today confirmed the presence of the plum pox virus (PPV) on a plum tree sample from Southwestern Michigan.

Plum pox is a viral disease of stone fruit species that first appeared in the United States in Pennsylvania in October 1999 and most recently in New York in July 2006. The plant virus does not pose any human health risks. This virus is also found in Canada.

The plum tree sample was collected at the Southwest Michigan Research and Experiment Center (SWMREC), a Michigan State University facility, located near the Benton Harbor-St. Joseph area. The samples were collected as part of state surveillance surveys and underwent preliminary testing at the Michigan Department of Agriculture where researchers obtained positive results.

The plum pox strain identified in Michigan is the D strain of the virus--the same strain that is present in Canada, Pennsylvania and New York. The D strain of the virus is less virulent than other strains, does not infect cherry trees and is not seedborne. Because the strain is not seedborne, it is not necessary to regulate the movement of fruit to prevent the spread of the disease.

The Michigan Department of Agriculture, in cooperation with APHIS, has conducted extensive surveys for PPV since 2000. Survey specialists are currently surveying all 14,000 host trees at the SWMREC facility. Following the completion of this survey, APHIS and MDA will expand surveillance efforts to include host trees within two miles of the center.

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PPV is the cause of a serious plant disease, affecting a number of species, including peach, nectarine, apricot and plum. Several aphid species can serve as carriers of the virus. The virus stays viable in the aphid’s mouth parts for a period of approximately one hour and most aphids can generally transmit infection several hundred meters from the initial source plant. (Animal and Plant Health Inspection Service news release)

2. Fungicide Use on Plasticulture Strawberries in the Fall

Many growers have received their tips and begun the rooting process. Disease control is a primary concern at this time. With recent disease problems, it is natural to want to spare no expense in protecting the crop. However, we should try to direct our efforts where we know they should be most effective. If we followed an intensive spray program from the time the tips are stuck until winter, and then resumed it in the early spring and continued through harvest, we would have a tremendous bill for chemicals. Plus, we would be setting ourselves up for poorer disease control and wasted spray applications in the future due to the development of resistant strains of the pathogens!

There are three main diseases with which to be concerned: Botrytis, Phytophthora crown rot, and anthracnose.

Botrytis: With the frequent misting needed to root the tips, Botrytis can get started in the plug bed. Occasional spraying with a Botryticide in the plug bed is justified. Care must be taken not to deplete your allotted number of applications per year. Many fungicides have only 4 allotted sprays per season (year). Keep in mind that crunch time for Botrytis is in the spring, during bloom. Try to include some chemistries that you may not normally use for Botrytis control, such as Rovral or Topsin M. Rovral can be used one time per year. Topsin M can be used several times, but is often good for only one application before resistance becomes a factor. Allow the spray to dry thoroughly before turning the mist system back on.

It has been speculated that a Botryticide application in the field during the fall will help reduce fruit rot the following spring. I am not aware of any research that indicates that to be true. The idea has merit in theory, since the fungus gets a foothold at that time. However, Botrytis infects young, emerging leaves throughout the fall during favorable weather conditions. To reduce this activity, fungicides would have to be applied regularly. This would be another recipe for excessive fungicide use.

Phytophthora: If you know your ground is infested with the Phytophthora crown rot fungus, make one application of Ridomil after planting and another one in the early spring. If the ground has not been known to be infested and your plants are Phytophthora-free, you do not need to make this application.

Anthracnose: Your guide for fungicide use here should be whether anthracnose was found on the plants

http://eppserver.ag.utk.edu/Extension/fpn/fpn082406.htm (2 of 5)12/6/2012 4:05:07 AM Fruit Pest News either before or after transplanting to the field. If it was not, and the plants did not come from an infested lot, fungicides would not be needed during the fall. For anthracnose-infested fields, a minimal fungicide spray program can be followed, but keep in mind the allotted number of applications for each product. I am not aware of any research that indicates that fall applications of fungicides are beneficial in anthracnose control. I do know, from my research efforts, that anthracnose fungicides, applied during the summer for control of anthracnose runner rot in matted row strawberries, do not work well.

In summary, usage of fungicides in the fall on plasticulture strawberries is faced with two main problems: (1) a lack of proof that they are beneficial and, if so, how best to use them; and (2) consumption of valuable allotted fungicide applications, and associated increased risk of resistance development. Don't feel guilty about not spraying your crop..... Sometimes, it's the best thing you can do!!!(SB)

3.. Apple Fruit Rot Diseases

Fruit are entering their period of highest susceptibility to rot diseases. It may be worthwhile to review the major rot diseases of apple fruit, so that your spray program can be adjusted accordingly, if needed.

Black rot of fruit is characterized by dark, firm rots that are not sunken. As the lesions enlarge, they may produce a series of concentric rings alternating from black to brown. Fruit infection can occur early in the season, as soon as the bud scales begin to loosen. This early infection usually involves the sepal, and results in blossom-end rot later in the season. The black rot fungus also causes frogeye leaf spot and limb cankers.

White rot, sometimes called Bot rot, can be destructive on certain varieties such as Golden Delicious, on which small, tan, developing lesions are often surrounded by a red halo. When the fruit rot under warm conditions, as they usually do, the rot is tan to light brown, soft, and watery. Under cooler conditions, rotted areas are usually firmer and deeper tan, and resemble black rot. White rot fruit lesions extend in a cylindrical pattern to the core of the fruit.

Bitter rot fruit infections are circular, sunken, and brown in color. When developing lesions reach about the size of a quarter, concentric rings of spores are produced around the center of the lesion. Under moist, humid conditions, the spore masses appear creamy to pink to orange in color. Decay lesions extend in a cone-shaped pattern toward the core, as opposed to the cylindrical shape of white rot lesions. Numerous infections can occur and losses can be extensive in warm, wet weather.

Apple fruit rots are managed by good pruning practices to improve spray coverage and drying time. Either remove the prunings from the orchard or mow them with a flail mower. (SB)

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4. Grape Downy Mildew

Downy mildew often becomes very noticeable in vineyards in late summer and fall. This disease causes direct yield losses by affecting flower and fruit clusters. Indirect losses can result from premature defoliation of vines due to leaf infections. Be wary of premature defoliation of vines after harvest, because this predisposes the vines to winter injury.

Summer spay programs are needed even where a conscientious program was followed during the spring. You can't rest! Springtime symptoms of this disease often go unnoticed. Furthermore, spores of this fungus are capable of riding on air currents for some distance, so they can enter a vineyard during the summer.

Leaf symptoms can vary depending on the age of the leaf when infected, and the temperature. Leaf spots can be large and yellow, with indistinct margins, or small, angular, yellow to necrotic, and limited by the veins. In late summer and fall, most leaf infections appear as the latter. Older leaves become infected first, and the disease spreads towards the leaves at the ends of the canes. Leaves eventually become dry and brown and fall from the cane. Such defoliation reduces sugar accumulation in fruit and decreases hardiness of overwintering buds.

Shoot tips, tendrils, and blossom clusters may also be infected. Young berries are very susceptible, appearing grayish when infected and covered with a downy felt of fungus sporulation. Although berries become less susceptible as they mature, infection of the rachis (the main cluster stem) can spread into older berries. Berries infected at this time generally do not soften or show a downy growth; instead, they become brown and shatter easily.

The downy mildew fungus overwinters primarily in fallen leaves. Survival is best on the soil surface and is enhanced by wet winters. The overwintering spores germinate in water in the spring to produce a spore that disperses to plant tissue by splashing rain.

Maneb, mancozeb, Ridomil Gold Copper, and Ridomil Gold MZ are excellent downy mildew fungicides, but have 66-day preharvest intervals. Among the fungicides used during the summer on grapes, captan, Abound, fixed copper, Pristine, and ziram have the best activity against downy mildew. A 10 to 14-day spray interval should be followed until the harvest restriction is reached. Susceptible varieties should be sprayed after harvest also, so as not to incur the vine-weakening effects of defoliation by downy mildew. (SB)

5. Using Sulfur on Cucurbit Crops

With the resistance problems that have occurred with fungicides for control of powdery mildew of

http://eppserver.ag.utk.edu/Extension/fpn/fpn082406.htm (4 of 5)12/6/2012 4:05:07 AM Fruit Pest News cucurbits, sulfur should be getting increased attention. Not all sulfurs are created equal, especially with regard to crop clearances, as shown in the table below. Some crops may not have clearances, not because of phytotoxicity problems, but because of company decisions not to pursue those markets. Follow the product label for crop clearances and rates to use. Cucumbers and cantaloupes tend to be sensitive to sulfur in hot weather and may burn. Do not apply sulfur to cucumbers and cantaloupes if temperatures exceed 90 degrees, unless the variety is sulfur tolerant. (SB)

Sulfur Products for Cucurbits

Cucumber Melons Squash Pumpkins Product

Microthiol Disperss 2-4 lb -- 5-10 lb 5-10 lb (Cerexagri) 4-6 lb 6-15 lb 4-10 lb -- Thiolux (Syngenta)

Sulfur DF (Wilbur- 2-6 lb 5-25 lb 5-10 lb -- Ellis)

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

Contacts:

Steve Bost, Professor and Extension Plant Pathologist [email protected]

Frank Hale, Professor and Extension Entomologist [email protected]

Both authors available at: 615-832-6802 fax 615-781-2568 Plant and Pest Diagnostic Center 5201 Marchant Drive Nashville, TN 37211

Copyright 2006 The University of Tennessee. All rights reserved. This document may be reproduced and distributed for nonprofit educational purposes provided that credit is given to University of Tennessee Extension.

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