Fruit Pest News

Volume 3, No. 10    May 13, 2002

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. Peach Pest Management - Diseases

    2. Apples: Phyto Reminder for Quadris/Abound

    3. Grapes: Critical Period for Black Rot Management

    4. Small-Batch Preparation of Pesticides

1. Peach Pest Management - Diseases

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. Green fruit rot is not often seen, but the prevalence of frost-killed, clinging fruit this year could result in infected fruit. Scout these and thinned fruit on the ground for 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 fruit is present. These materials will also control scab. (SB)

2. 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 McIntosh 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, McIntosh, Molly's Delicious, Kent, Spartan, Summer Treat, and Summared. Also related to McIntosh are Bancroft, Empire, and Janamac. Note: Azoxystrobin injury on Gala could be mistaken for Glomerella leaf blotch, to which Gala is also susceptible. (SB)

3. Grapes: Critical Period for Black Rot Control

The pre-bloom to early bloom spray is a very important spray for controlling black rot as well as the other major grape diseases. Inoculum has had time to build and the plant tissues are very susceptible. During this time, the use of one of the sterol inhibitors (Nova, Elite, Bayleton, etc.) is recommended because of their locally systemic activity. The cool, wet weather of late may create a Phomopsis problem. Since the sterol inhibitors are not effective against Phomopsis or downy mildew, an effective product such as mancozeb or captan should be applied with the sterol inhibitor. A strobilurin fungicide (Abound, Flint, Sovran), applied alone, would serve the same purpose except for Phomopsis control (only moderate). On bunch rot susceptible varieties, use Rovral, Vanguard, or Elevate when bloom begins.

Since this is a critical spray, take care to obtain good coverage of all foliage and developing fruit. Reduce the tractor speed, spray every row middle, increase volume, and use full label rates. (SB)

4. 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)

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