Fruit Pest News

Volume 6, No. 11  May 31, 2005

A weekly, 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.

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. Observations

    2. Brambles: Raspberry Bushy Dwarf Virus Found in Ohio

    3. Effect of Rain on Fungicide Wash-Off

    4. Disease Infection Periods for the Month of May

        4.a. Grape Black Rot, Botrytis Bunch Rot, and Downy Mildew

        4.b. Grape Powdery Mildew

        4.c. Apple Sooty Blotch and Flyspeck

1. Observations

Dry and cool. The month of May has been very dry, with only about an inch of rain in the Nashville area. That should change over the next few days, with rainy periods in the forecast.

Those of you who grow tomatoes, pepper, or tobacco may have a bad experience with tomato spotted wilt virus this summer. Dry springs often correlate with this disease, probably because of their effects on the natural predators of the thrips vector of the virus. Hopefully, the wet weather we had in April will negate those effects.

First reports for the year reported during the past week: pecan phylloxera -- May 23, Decatur County (west); southern stem blight of tomato -- May 26, Rutherford County (central); grape phylloxera -- May 27, Shelby County (west); Asian ambrosia beetle on muscadine -- May 27, Lawrence County (central). (SB)

2. Raspberry Bushy Dwarf Virus Found in Ohio

Raspberry bushy dwarf virus (RBDV) was found in 21% of leaf samples collected from eight raspberry fields in Ohio in 2004. This was the first finding of this virus in Ohio. RBDV has been a problem in Pacific Northwest bramble production for several years. The survey in Ohio was undertaken because of a decline in plant vigor and survival in several fields. Symptoms had included small, dry, and mishapened berries, foliar chlorosis, and premature dying of fruiting canes before or during harvest. This decline generally had been attributed to winter injury.

It may be advisable to avoid the purchase of raspberry or blackberry plants from the Northwest because of the prevalence of RBDV. This is a pollen-transmitted virus that can easily spread once introduce into a field. There are no resistant blackberry or black raspberry varieties. The red raspberry cultivars Willamette, Haida, Heritiage, and Latham are reported to be resistant to the disease. (SB)

3. Effect of Rain on Fungicide Wash-Off

If rain is forecast on or around the day you plan to make a fungicide application, it is best to spray before the rain, rather than after. Disease-control products work best in a preventive mode. They need to be in place, working to protect the plant tissues while they are wet. But that begs the question, "Just how durable are those protective covers? How soon do I need to re-apply a fungicide after a rain?" The following article was written for apple growers by Dr. Jim Travis, Pennsylvania State University plant pathologist. The guidelines he provides should apply to all crops, although the example products may not apply. (SB)

If you are using protectant fungicides, you need to consider the effect of rain on wash-off of the materials. The strobilurin (Sovran, Flint) and sterol inhibitor (Nova, Procure, Rubigan) fungicides are absorbed into the leaf and fruit tissue after application (once the residue has dried) and are not affected by rain wash-off. The protectant (Dithane, Manzate, Penncozeb, Captan, Ziram, Thiram, Carbamate) fungicide residues can be affected by rain. A general rule-of-thumb for the effect of rain on washing off protectant fungicides follows.

• Less than one inch of rain since the last spray will not significantly affect residues.

• One to two inches of rain will reduce the residue by 1/2. Reduce the number of days until the next spray by 1/2. Example: If the next spray is scheduled for 8 days, then reduce that interval to 4 days.

• Over two inches of rain since the last spray will remove most of the spray residue. Renew the fungicide deposit (re-apply) as soon as possible.

This rule has been used for many years to provide growers with general guidance. Newer protectant fungicide materials may be less subject to wash-off, but information is limited.

4. Disease Infection Periods for the Month of May

Infection periods for the month of May are presented below. The weather data upon which this information is based was collected on a Spectrum WatchDog system placed at the plant diagnostic center in Nashville. This information may familiarize you with this method of timing the application of fungicides in an effort to improve control while reducing the number of sprays. Each disease is based on a different "model," or system of predicting infection. (SB)

4.a. Grape Black Rot, Botrytis Bunch Rot, and Downy Mildew

* Black rot infection risk begins at a value of approximately 1.00.

4.b. Grape Powdery Mildew

With this predictive model, the infection risk for a given day is based on previous days' conditions. Once the infection risk develops to the heavy category, it can move down to the medium category only after several days of very cool or very hot weather. This month, that never happened, so the risk remained heavy throughout the month. The index increases by 20 with each day having 6 hours between 70 F and 85 F. The index decreases by 10 on days with less than 6 hours between 70 F and 85 F, and on days with a maximum temperature greater than 95 F. The risk is determined by the following index values:

The risk for infection by powdery mildew conidial spores has been heavy throughout the month.

4.c. Apple Sooty Blotch and Flyspeck

Accumulated wetting hours (AWH) can be used to predict the appearance of sooty blotch and flyspeck on apple fruit, and the time at which a fungicide spray is needed for control. With this method, unnecessary applications of fungicides for control of these diseases can be avoided. Research has shown that sooty blotch and flyspeck will appear after about 260 - 300 AWH (wet periods caused by rain or dew) following the biofix of 10 days after petal fall. This model can thus extend the time between the first and second cover sprays. Topsin M, Flint, and Sovran appear to be effective in arresting fungal development if used within 175 AWH after infection.

The biofix (when you begin counting the hours) of 10 days after petal fall occurred on May 10 (plant development was late because of a very cool spring). Since then, 92 AWH have accumulated at this location (wet periods of less than 4 hours are not included). (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

Plant and Pest Diagnostic Center

5201 Marchant Drive

Nashville, TN 37211

Copyright 2005 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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