Fruit Pest News

May 21, 2001

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 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. Damage from April Freeze

    2. Grapes: Spray Program Strategy for Black Rot

    3. Grapes: Time for Botrytis Sprays

    4. Apples: Shoot Blight Prediction

    5. Strawberries: Anthracnose Reports

1. FRUIT CROP DAMAGE FROM APRIL FREEZE

The extent of the damage from the cold weather in mid-April is becoming apparent. With temperatures hovering around the freezing mark in much of the state, the amount of damage varied from one part of the state to another and within orchards. "The freeze damage was mostly in East Tennessee, with the extent of the damage ranging from a nearly total loss to a partial crop," said David Lockwood, a fruits specialist with the University of Tennessee Institute of Agriculture. "Damage extended as far west as Carthage in Middle Tennessee," he said, "but it wasn't cold enough to damage fruit trees in southern Middle Tennessee and most of West Tennessee." Mutsu and Jonagold appear to be the varieties hardest hit by the freeze.

Many wine grapes in East Tennessee also were damaged. "Some fruit for East Tennessee wineries may have to be trucked in from Middle Tennessee," Lockwood said. "But transportation makes wine-making more costly." (SB)

2. SPRAY PROGRAM STRATEGIES FOR BLACK ROT OF GRAPE

Black rot is the most destructive disease affecting grapes in Tennessee. In some of the literature and on some fungicide labels, you will see two types of strategies for applying fungicides for black rot control. One is referred to as protective or preventive. It involves spraying on a set schedule such as every 7 to 14 days. The spray interval chosen by the grower would be determined by the amount of rain: More frequent sprays would be needed in rainy weather, because most disease organisms are favored by wet conditions, and because rainfall can remove some of the fungicide residue.

The other strategy for applying black rot fungicides is referred to as a postinfection or eradicative schedule. With this method, certain systemic fungicides such as Bayleton or Nova are applied within 72 hours after the beginning of an infection period. An infection period occurs when moisture is present long enough at a certain temperature for the fungus spores to germinate and enter the plant tissue. The table below shows how long the tissue must remain wet at each of several temperatures for infection to take place.

The idea behind the postinfection method is to attempt to reduce the number of fungicide applications and to make the applications only when they are truly needed. Sounds good, but wait. Notice in the table above that infection can occur due to dew; that is, at temperatures typical of springtime nights, black rot infection can occur in one night because of the moisture provided by dew. I monitored wet periods and temperature in a Sumner County vineyard one year, and found that infection periods occurred most nights. I compared two schedules in the trial: protectant vs. postinfection. With so many infection periods, the postinfection schedule saved only one fungicide application (8 for the postinfection, 9 for the protectant).

Another consideration is that control of Phomopsis cane and leaf blight and downy mildew with the postinfection schedule can be weak. Conditions required for infection by these two diseases can differ from those for black rot. Furthermore, there are no eradicative fungicides for these diseases.

While a protectant approach for black rot is preferred, the eradicative technique is needed at times. Black rot infections can occur if the weather becomes rainy some time after your last fungicide application (e.g. 7 days or more). In that case, a black rot fungicide with good eradicative activity should be applied as soon as possible. Nova, Elite, or Bayleton would be good choices. The strobilurins (Abound, Flint, Sovran) have some eradicative activity, but should be used primarily as protectants. (SB)

3. TIME FOR BOTRYTIS SPRAY IN GRAPES

Early bloom is the time for the first application of a Botrytis fungicide. Botrytis bunch rot can be a problem in tight-clustered French hybrid or Vinifera varieties. Two recently labeled fungicides should help us in our efforts to prevent bunch rot. Vangard 75WG (Novartis) and Elevate 50WG (TomenAgro) represent two new classes of chemistry, so they will help us in resistance management.

Vangard is used at 10 oz per acre at early bloom and at berry touch, veraison, or preharvest (20 oz per acre per year, 7-day PHI). See the label for precautions for use near aquatic areas. Elevate is used at 1 lb per acre at early bloom, bunch pre-close, and veraison, up to and including the day of harvest (up to 3 applications). Rovral 50WP may be applied at 1.5 to 2 lb per acre at early bloom, prior to bunch closing, veraison, and preharvest (7-day PHI). Although the Rovral label allows 4 applications, it is suggested that no more than 3 applications be made, in light of resistance problems that have occurred in other areas. Benlate can be used at 1 to 1.5 lb per acre at early bloom and repeated 14 days later. (SB)

4. SHOOT BLIGHT IN APPLES

Predicting the time of appearance of shoot blight symptoms appears to be trickier than blossom blight. MaryBlyt, accurate to the day in predicting infection of blossoms and the appearance of blossom blight symptoms, missed the shoot blight prediction. That is probably because the program assumes that certain sucking insects allow entry of the bacteria into the shoot leaves. Recent research indicates that wind damage is more important than insects in allowing the bacteria to enter the shoot leaves. Using a wind storm here on May 7 as the biofix (beginning point), MaryBlyt accurately predicted May 16 as the date of first symptoms. MaryBlyt placed the degree day countdown in the "Trauma Blight" column. (SB)

5. STRAWBERRY ANTHRACNOSE REPORTS

Two severe cases of anthracnose fruit rot in plasticulture strawberries were reported to me last week. The infections apparently were incited by the showers and storms of two weeks ago, although one of the locations reported very little rain. This week's showers will hamper control efforts. Our most effective labeled control product is Quadris, for which Tennessee has a Section 18 label. Where anthracnose is present, apply Quadris every 7 days. Quadris should be alternated with captan or thiram on a twice weekly basis (Quadris followed by captan or thiram 3-4 days later), or tank mix them once per week. The Quadris label does not prohibit tank mixing, but rotation to an unrelated fungicide such as captan is still necessary after 3 Quadris applications, whether tank mixed or not. Use high-pressure sprayers and aim for at least 100 gallons of water per acre for broadcast sprays, in which row middles are sprayed.

One case of anthracnose crown and root rot, confirmed by lab isolation as Colletotrichum gloeosporioides, occurred in a matted-row field. The problem occurred in both first-year and second-year plantings. The grower did not report fruit rot, but this species of anthracnose is capable of causing fruit rot like C. acutatum. (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, 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