Fruit Pest News

Volume 4, No. 9   May 12, 2003

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. Current Conditions

    2. Strawberries: Rhizoctonia Root Rot

    3. Small-Batch Preparation of Pesticides

    4. Peach Pest Management - Diseases

    5. Apples: Phyot Reminder for Quadris/Abount

    6. Apples: Trauma Blight Fire Blight

    7. Grapes: Time for Botrytis Spray

1. Current Conditions

Last week's severe weather took its toll on structures and commodities. Storms that included high winds, heavy rainfall, hail, and tornadoes occurred during much of the week. Fields had problems with standing water, erosion, and debris. Rainfall totals varied from 2 to 11 inches across the state during the week, with most areas receiving 5 to 8 inches. Fruit crops suffered from hail damage. Apples may also experience fire blight as a result of the wind damage, if the bacterial population was high. Strawberry fruit will suffer from flavor dilution due to the excess water, if the berries are harvestable at all. Phytophthora diseases can be expected to be worse this year. Hopefully, things will dry out and we can get a better assessment of the damage. This week's weather appears to be drier and cooler. (SB)

2. Rhizoctonia Root Rot of Strawberries

The diagnostic lab just received two samples of strawberries with Rhizoctonia root rot. The stress of the fruiting season tends to make these infections express themselves.  The disease may appear as a gradual stunting and decline of plants or as a rapid wilting and death of leaves of vigorous plants. Affected plants usually appear in a small area, but the symptoms can expand in the field rapidly. Infected roots have dark brown lesions, and a brown discoloration can appear inside the crown and even the petioles. It is thought that root infections by Rhizoctonia are favored by cool temperatures, while crown and petiole infections are favored by higher temperatures.

The two samples we received were from commercial, matted row plantings. One of the fields was in its first fruiting year and showed the acute type of symptoms. In Virginia, it has been noted that Rhizoctonia root rot always appears in old strawberry land not rotated to other crops for at least four years between strawberry crops. Lack of crop rotation undoubtedly will lead to a buildup of this soil-borne fungus. Virginia specialists have observed that this disease can occur in matted row as well as plasticulture production and in fumigated as well as non-fumigated soil. Fumigation can make matters worse by destroying the fungus' natural enemies and allowing the surviving Rhizoctonia to flourish.

Control is by prevention only. First, send suspect plants to the plant diagnostic lab so that a positive identification can be made. Where Rhizoctonia root rot is found, reduce plant stress by irrigating during dry weather. Follow a crop rotation plan, allowing at least 4 years between strawberry crops. The biological control fungus Trichoderma harzianum (T-22, RootShield, PlantShield) has been found to be effective against this disease when used as a preplant soil treatment. (SB)

3. Small-Batch Preparation of Pesticides

When preparing small quantities of a spray mix (e.g. 1 gal.), it is often convenient to use a tablespoon to measure the product. That's fine if the product is a liquid. Any liquid pesticide mixed at 1 pint per 100 gallons of water is always going to be 1 teaspoon per gallon, because both measures are volumes. The problem comes with dry products (wettable powders, dry flowables, water-dispersable granules).

Dry products have different densities, so it is not accurate to convert a weight measure into a volume measure (e.g. tablespoon), unless you know how much that volume weighs. The table below illustrates the range of densities of some common fruit fungicides. You can see the error that would ensue if you assumed, say, that 2 tablespoons per gallon of any dry product was equal to 3 pounds per 100 gallons.

Once you have weighed a level tablespoon of a dry product, you can use that tablespoon to measure the product, because it is then a unit of weight. (SB)

4. Peach Pest Management - Diseases

Watch for certain indicators that can signal upcoming problems. You may need to adjust your control program, such as shortening the spray interval or changing to another control product. You're practicing IPM and may not even know it!

Bacterial spot. Most diseases are favored by rainy weather, but some react more explosively to it than others. Bacterial spot can become epidemic quickly because of the ability of the bacteria to reproduce rapidly and because of their ability to exist epiphytically on the various surfaces of the tree. The number of overwintered shoot "blacktip cankers" is not necessarily a good indicator of the disease potential, because the epiphytic population represents an invisible source of infection. The level of bacterial spot in previous years is probably the best indicator of the disease potential in a block. On susceptible cultivars, successful control of bacterial spot requires a program involving dormant pruning of cankers and use of copper sprays, early-season sprays of copper or Mycoshield, and some help from Mother Nature (dry weather).

When using Mycoshield, consider that it does not weather well. Because of its short residual activity, Mycoshield is most effective when applied shortly before rain, before it has a chance to inactivate. Try to apply within 24 hours before a wetting event, but allow time for the spray to dry. Copper sprays can be used at this time of year, but leaf damage can occur. If you choose to use copper now, use low rates of liquid coppers such as Tenn-Cop 5E (4-6 fl oz/acre) or Copper-Count-N (4-6 fl oz/acre) in at least 100 gal water per acre.

Peach scab. As with most diseases, the risk of infection can be determined by the severity of the disease in previous years. With peach scab, disease potential can also be assessed by examining fruit-bearing shoots for the presence of lesions. Scab lesions on twigs are brown, circular to oval in shape, and best seen on the parts of the shoot that have not yet turned from green to brown. If scab potential is considered low, sulfur should be adequate for control. Remember that sulfur is not very rainfast. Scab control is not needed within 40 days of harvest, because that is the length of time needed for symptom development after infection.  

Green fruit rot. This is a good indicator for brown rot, as both diseases are caused by the same fungus. Clinging green fruit that has been damaged by frost and thinned fruit on the ground may be infected and serve as an indication of brown rot pressure. Insect- or wildlife-injured fruit is also susceptible to brown rot and serves as a good indicator. Adjust cover sprays or initiate pre-harvest brown rot sprays early when disease pressure is high. Use captan or Abound if infected green fruit is present. These materials will also control scab. (SB)

5. Phyto Reminder for Quadris/Abound

With azoxystrobin (Quadris, Abound, Heritage) receiving registrations for so many commodities, its danger to certain apples bears repeating. Azoxystrobin is very phytotoxic to Macintosh and related apple varieties. When spraying strawberries or other crops with an azoxystrobin product, be careful not to allow any drift to nearby apples. Do not even use a sprayer for apples that has ever been used to apply azoxystrobin. Even trace amounts can burn the leaves. Before selling a sprayer that has been used to apply azoxystrobin, clean it out thoroughly, using a tank cleaner. The following varieties have been reported as sensitive to azoxystrobin: Akane, Bromley, Cortland, Cox/Cox's Orange Pippin, Discover, Gala, Grimes, McCoun, Macintosh, Molly's Delicious, Kent, Spartan, Summer Treat, and Summared. Also related to Macintosh are Bancroft, Empire, and Janamac. Note: Azoxystrobin injury on Gala could be mistaken for Glomerella leaf blotch, to which Gala is also susceptible. (SB)

6. "Trauma Blight" Fire Blight

Last week's storms remind us of the bad things that can happen to trees to cause"trauma blight." This is fire blight that results from infections associated with injuries due to late frosts, hail, or high winds. Expect trauma type fire blight to be most severe when the bacterial population is high. While free water on the surfaces of leaves is likely during a frost or with hail, infections do occur in the absence of rain under high wind conditions that tatter the foliage. Even here, rain increases the severity.

Action to take: If a trauma event occurs, a streptomycin application should be made within 24 hours, if fire blight symptoms are present in the orchard. The application should be made even if the foliage is still wet. If you think the fire blight bacterial population is high (the block has a history of fire blight and the weather has been constantly warm), apply the streptomycin even though no symptoms are present. Note: It is only after a trauma event that we recommend the use of streptomycin after bloom. It is not recommended for routine control of shoot blight. This is because it is not effective for control of shoot blight, and repeated use of this valuable product can cause resistance to it to develop in the fire blight bacterium. (SB

7. 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 and Elevate 50WG 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. Benlate can still be used if it can be found. (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