Fruit Pest News

Volume 9, No. 12  June 11, 2008

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. Brown Rot of Peach and Other Stone Fruits

    2. Apple: Summer Disease Control Program

    3. Powdery Mildew of Apple

    4. The Apple Leaf Blotches: Necrotic, Alternaria, and Glomerella

    5. Wine Grapes: Time for Botrytis Spray

1. Brown Rot of Peach and Other Stone Fruits

The two-week period prior to harvest is a critical period in the life of a peach. This is the period in which it is most susceptible to brown rot, the most important disease of peaches in Tennessee. Fungicide sprays just before and during the harvest period are very important. Because of the threat of resistance development in this fungus, use fungicides from two classes: the sterol inhibitors (Indar, Elite, Orbit) and the strobilurins (Pristine, Abound). Minimize the use of these fungicides, making no more than two applications of either class. Make at least two applications of brown rot fungicides during the pre-harvest period, beginning no later than two weeks prior to the beginning of harvest (you need to know when to expect harvest to begin). Make an additional one to two applications during harvest. Adjust spray intervals as needed based on weather conditions, inoculum carryover, and the amount of physical injury to the fruit.

Injuries to fruit that can increase brown rot problems include hail, frost cracks, bird feeding, and insects such as Oriental fruit moths, Japanese beetles, green June beetles, hornets, and wasps. Breaks in the fruit skin make it easier for the fungus to enter and begin the rot process. Wet, warm weather aids dispersal and germination of the fungal spores. The spores come mainly from mummified fruit from the previous year, and the spores are carried to fruit by wind, rain, and certain insects.

Some brown rot is not apparent at harvest and develops in storage. Storing the fruit in a cooler after harvest slows the development of infections. Try to avoid injuries to the fruit during the harvesting operation, as post-harvest brown rot can begin at these sites. (SB)

2. Apple: Summer Disease Control Program

Your summer disease control program is, no doubt, well underway. The so-called summer diseases take a greater toll on Tennessee apple production than the spring diseases. The summer diseases are fueled by a continuing source of overwintered inoculum, and there are fewer effective fungicides from which to choose. Furthermore, it is more difficult to obtain good spray coverage during the summer than during the spring because of more dense foliage, fruit clustering, and limbs drooping under heavy crop loads.

Your choice of fungicides will depend to some extent on what diseases have historically been a problem in your orchard. If bitter rot has been a problem, captan should be a major component of your summertime spray program. Flint also performs well against bitter rot. If white rot or black rot has been a problem, captan or Topsin M should be relied on heavily. For sooty blotch and flyspeck, be sure to include Topsin M or a strobilurin (Flint, Sovran, Pristine). Strobilurins are also a valuable component of a program if scab is still active. In Golden Delicious blocks, it is a good idea to include ziram or thiram to help protect against necrotic leaf blotch (see article below). Mancozeb provides good control of most summer diseases, but cannot be used within 77 days of harvest when used at the 3/4 lb per 100 gallon rate.

Remember that management of diseases with fungicides is improved if cultural practices that reduce inoculum and enhance coverage are used. (SB)

3. Powdery Mildew of Apple

Powdery mildew will continue to spread as long as there are new leaves developing on apple trees. When terminal growth stops, the potential for spread of powdery mildew will decrease. On susceptible varieties, apply effective mildewcides such as the sterol inhibitors, the strobilurins, or sulfur every 10 to 14 days until terminal growth stops. (SB)

4. The Apple Leaf Blotches: Necrotic, Alternaria, and Glomerella

During the summer in Tennessee, three apple diseases can occur, all having similar names and symptoms. Let's compare these diseases and their controls.

Necrotic leaf blotch is actually not a disease, but a physiological disorder. It occurs on Golden Delicious and its sports. The affliction often occurs in waves during the summer. It tends to be more severe when hot, sunny weather follows several days of mild, cloudy weather. Large, irregular-shaped, necrotic (brown) spots appear suddenly on green leaves (click here to view an image). Most affected leaves soon turn yellow and fall from the tree. Defoliation can be significant. Although necrotic leaf blotch is not caused by a fungus, severity is decreased by a regular fungicide program (see below).

Alternaria leaf blotch, caused by the fungus Alternaria mali, affects Red Delicious and related varieties such as Empire. Alternaria leaf blotch is a problem in the higher elevation areas of Tennessee. Symptoms (click here) include small, circular necrotic spots on the leaves. The leaf spots may have a purplish margin and, thus, resemble frogeye leaf spot and captan injury. Alternaria leaf spot is aggravated by European red mites. Control consists of mite control, fungicide spray programs, and adequate tree spacing and other cultural practices that enhance drying conditions.

Glomerella leaf blotch is very limited in distribution. Symptoms (click here) resemble necrotic leaf blotch of Golden Delicious. However, Glomerella leaf blotch has only been found on Gala and Ozark Gold. Also unlike necrotic leaf blotch, Glomerella can infect the fruit, causing a bitter rot type of infection. Appropriate fungicides for control of Glomerella are provided below. As with Alternaria leaf blotch, Glomerella is most severe where tree spacing or branch spacing is inadequate, or other conditions exist that interfere with drying conditions. Note: Necrotic areas on leaves of Gala can also be caused by Abound injury.

*The EBDC fungicides have a 77-day preharvest interval.

As you can see, the 3 blotches have different cultivar spectra and the chemical control specifications are different in some cases. If you find any of these blotches, we would be interested in knowing about it, and would be glad to assist you in selecting control products. (SB)

 5. Time for Botrytis Spray on Wine Grapes

Bunch closing, the time when the berries start to touch, is coming soon for most varieties. A Botrytis fungicide should be applied before bunch closing on tight-clustered French hybrids such as Vignoles and Seyval, and most vinifera varieties, especially Pinot Noir, Riesling, and Chardonnay. Also, removal of leaves around clusters before bunch closing has been shown to reduce losses to Botrytis, by improving the drying conditions in the cluster.

The lineup of bunch rot fungicides has become impressive. It includes Rovral, Vangard, Elevate, Endura, and Scala. Endura has powdery mildew activity, but the others are effective only against Botrytis. You need to apply other appropriate fungicides for control of black rot, bitter rot, the mildews, etc., all of which will continue to be active throughout the summer. It is also important to remember that these Botrytis fungicides are prone to resistance development in the pathogen population. It is therefore recommended that none of these materials be used more than 3 times per year. The Vangard label limits its use to 2 applications per year, at the 10 oz/A rate. Try to use 3 products for Botrytis control, and alternate them. (SB)

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, Professor and Extension Entomologist

fahale@utk.edu

Both authors available at:

615-832-6802

fax 615-781-2568

Soil, Plant and Pest Center

5201 Marchant Drive

Nashville, TN 37211

Copyright 2008 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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