Fruit Pest News

June 4, 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. Apples: Fire Blight Update

    2. Apples: Necrotic Leaf Blotch of Golden Delicious

    3. Strawberries: Disease Control after Harvest in Matted Row

    4. Strawberries: Phytophthora Crown Rot Control

1. FIRE BLIGHT UPDATE

Fire blight is especially noticeable in many areas now. The severity of fire blight this year illustrates the role of temperature during the bloom period in determining fire blight. The weather was generally warm and dry during bloom. As you can see, wet weather is not needed. The key determining factor is temperature. Dews provide all the moisture needed for infection if the temperatures are in the right range. And the dews do not need to be heavy ones. A moderate dew is all that is required. Remember that, during the bloom period next year.

Fire blight spread will become less likely as the shoot growth slows down and hardens off. Until then, however, be mindful of the potential for "trauma blight" fire blight. This can occur when hail or high winds injure leaves or stems and allow the bacteria to enter. If such a storm occurs, apply streptomycin within 24 hours after the storm. (SB)

2. GOLDEN DELICIOUS GROWERS - PROTECT AGAINST NECROTIC LEAF BLOTCH

Necrotic leaf blotch can strike with great suddenness. A prevention program should be begun right away on Golden Delicious and its bud sports, because a remedy is not possible after the disorder appears. Although necrotic leaf blotch is not a disease, certain fungicides will help control it. In the past, we relied on the EBDC fungicides (Dithane, Manzate, Penncozeb, etc.) for control. However, with the current label restrictions (77-day PHI and 3 lb/acre maximum rate), the EBDC fungicides are no longer useful for managing necrotic leaf blotch. Dr. Turner Sutton of North Carolina State University recommends ziram or thiram, applied on a 10-14 day schedule from mid-June through early August. Tank mixing with captan will improve the control of certain other diseases. Foliar zinc sprays have also been tested for necrotic leaf blotch control. Dr. Sutton reports that Ele-Max Super Zinc, applied at 1 pint/acre with captan in 3 to 4 cover sprays has been effective, although other zinc oxide products may be just as effective.

Necrotic leaf blotch begins as broad necrotic areas on green leaves. The green turns to yellow and the leaves fall from the tree. The affliction can occur in several "waves" throughout the summer. It is usually most severe when a period of overcast, rainy weather is followed by sunny weather. (SB)

3. MATTED-ROW STRAWBERRY DISEASE CONTROL AFTER HARVEST

Harvest has wound down, and now it is time to take steps to produce a healthy bed of plants for next year's crop. Proper renovation is the most important step in producing a vigorous crop. When new growth emerges after renovation, fungicide applications may be necessary, especially in rainy weather. Anthracnose is the greatest threat, since this disease can prevent bed fill and can debilitate established plants. Fields that are known to be infested may require preventive fungicide applications. We have a Section 18 label for Quadris fungicide that is in effect until October 15. However, the runner phase of anthracnose, which affects stolons, petioles and crowns, is difficult to control. Efforts to control anthracnose on these vegetative parts of the plant may lead to disappointing results.

Nova fungicide is registered for use on strawberries. In my field trials, I have found Nova to be very effective against common leaf spot and Phomopsis leaf blight, but that it does not control anthracnose or Botrytis. Phomopsis can be a problem in hot weather, whereas common leaf spot is a problem on certain varieties in cool seasons. Below is a summary of the results I have obtained with field trials on the leaf spot diseases.

Please note the following:

• The Nova label states a rate of 2.5 to 5.0 oz/acre, applied at 14 to 21-day intervals. In a trial in which I used 4.0 oz/acre on a 7 to 10-day schedule, it caused the plants to become compact, with dark green leaves and reduced yields. The results in the table above were obtained with 2.0 oz/acre applied on a 7 to 10-day schedule. For leaf spot and leaf blight control, I would recommend the shorter end of the 14 to 21-day interval on the label.
• The Section 18 label for Quadris on strawberries is intended only for anthracnose control. However, when applied according to the label (6.2 to 12.4 fl oz/acre at 7 to 14-day intervals), it should provide control of leaf blight and leaf spot as shown in the table above. Also, do not make more than 3 consecutive applications of Quadris before alternating to another labeled fungicide for at least 1 application.
• The use of a surfactant would probably enhance the performance of any of the fungicides, especially against diseases of the stolons, which tend to be slick. (SB)

4. PHYTOPHTHORA CROWN ROT OF STRAWBERRY

This year, as in the past couple of years, there were a few cases of crown rot of strawberry, caused by the fungus Phytophthora cactorum. The question has come up, "What to do next year?" in fields in which crown rot occurred. First, let's look at a few of the basic facts involved in controlling this disease.

• The source of the causal fungus in a field is usually infected transplants or infested soil from a previous crop. It can also enter a field on soil brought into the field. It is not wind-borne.
• Phytophthora species tend to be long-lived in the soil, producing thick-walled spores called oospores that can persist in the soil for years.
• Another type of spore, called zoospores, have the unusual ability to "swim" in water and search for a host plant. Standing water is a haven for these spores, allowing them to move about and find a strawberry plant. Zoospores can also splash up onto above-ground parts of the plant in soil particles during rain.
• The fungus can enter the plant through roots, fruits, or stems. The common name for the fruit phase of the disease is "leather rot." (Leather rot does not necessarily lead to crown rot).

Control

Rotation. Because of the survivability of the fungus, and for general disease control, it is a good idea to locate next year's crop in a different field. However, many growers find it difficult to relocate their fields, because of lack of suitable fields and the need to locate near their water source.

Fumigation. Fortunately, soil fumigation will kill most of the Phytophthora and allow successful production of strawberries. However, the conventional method of fumigation in plasticulture strawberries is to fumigate in the row. This leaves the row middles untreated, and a potential source of infection of plants in the row by splashing of infested soil.

Row middle cover. A soil cover, such as ryegrass, for the row middles would be very beneficial for Phytophthora fields, by reducing splashing. A system should be followed that will provide soil cover through harvest.

Fungicide. In fields with a history of crown rot, the use of Ridomil Gold EC through the drip line would be recommended as a preventive. One application in the fall and one in the spring should be used. Foliar applications of Aliette 80WDG can also be used.

Soil drainage. Land leveling to eliminate swags would help prevent water from standing, as would ditching and tiling. Make sure the ends of rows middles running down slopes are not obstructed, and that drainage ditches are cut across rows that run along the contour of slopes.

What about double-cropping? Some growers plant cucurbit crops after strawberry harvest to take advantage of the plastic mulch and irrigation system in place. This should not be a problem. Cucurbit crops are subject to a serious Phytophthora disease, but it is caused by a different species, P. capsici. (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