Fruit Pest News

Volume 5, No. 12  June 1, 2004

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. Effect of Rain on Fungicide Wash-Off

    2. Strawberries: Matted-Row Disease Control after Harvest

    3. Peach Scab

    4. Apples: My Trees Have Fire Blight. What Do I Do Now?

    5. Insect Sightings

    6. Tree Fruit: Pheromone Trap Catches

1. 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.

2. 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. 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, even with effective fungicides such as Quadris and Switch, may lead to disappointing results. Resistant varieties such as Delmarvel do not need control measures.

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 Quadris label only lists anthracnose and powdery mildew control. However, when applied according to the label (6.2 to 15 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. Do not make more than 2 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)

3. Peach Scab

I have not yet seen peach scab symptoms, which usually begin appearing around this time.  Peach scab has a 40-day latent period, which means that symptoms do not appear for 40 days after infection. Control is not needed on varieties that are within 40 days of harvest, since any infections that occur will not have time to develop before harvest. This does not mean that early-maturing varieties will not have scab if it is not showing at this time (a 40-day latent period could be in progress). It just means that a spray at this time would not reduce scab on an early-maturing variety. Be sure to begin your brown rot sprays within 3 weeks of harvest for each variety. (SB)

4. Fire Blight: Managing Affected Trees

Following are some reminders for management of fire blight at this time of year. Efforts to limit secondary spread by cutting out fire blight strikes are most successful if the strikes are removed immediately after they appear. By this time in Tennessee, it is too late for strike removal to effectively reduce the rate of spread of the epidemic. Pruning of fire blight shoots during the growing season should be limited to light to moderate cases. Extensive pruning of heavily blighted trees is neither practical nor desirable, as the result would be the stimulation of new, succulent tissue, which is very susceptible. Young trees and high density, dwarf-tree orchards should be followed closely and strikes should be removed immediately, regardless of the severity of the outbreak. In such trees, the infection can spread internally to the rootstocks and cause tree death.

Classical recommendations for growing-season removal of fire blight strikes include making the cut 8 to 12 inches below the blighted area and sterilizing the pruning tool between each cut. We now know that this approach is not appropriate because of the systemic nature of the bacterial infection. If the infected shoot is associated with the main trunk or a major scaffold limb, you may want to try the "ugly stub" cut, described in the May 18 issue of Fruit Pest News.

Other practices to keep in mind: Excessive tree vigor should be discouraged. Use management systems that promote early cessation of tree growth without adversely affecting tree vigor. Maintain good control of sucking insects, as they can transmit the causal bacterium to healthy shoots. Remove from the orchard and destroy the fire-blighted shoots and limbs, since they can serve as a source of inoculum. If the amount cut cannot be removed quickly, there may be too much blight present for the cutting operation to be effective. (SB)

5. Insect Sightings

Leaf galls of the grape phylloxera were seen last week. There are both foliar and root forms of this insect. The leaf form overwinters as eggs beneath flaps of bark. In the spring, the eggs hatch and the asexual forms (crawlers) move to the top of the leaves which causes galls to form on the undersurface of the leaves. The females mature inside the galls. They then lay eggs inside the galls. These eggs were starting to hatch on May 27 in the Nashville area. These immature phylloxera will emerge from each gall via a hole in the upper surface of the leaf and crawl to new leaves and cause more leaf galls.

Newly emerged Japanese beetle adults were seen for the first time last Friday and over the weekend in Middle Tennessee. Daytime feeding from these pests will peak in June and continue into July and early August. They lay their eggs in turfgrass and the larvae (white grubs) feed on the roots of grass. Sprays with Sevin insecticide every 10 days or so may be needed to limit leaf skeletonizing on grapes, blackberry and raspberry while Imidan and Guthion can be used for this and other common pests in cover sprays on peach, nectarine and plum.

Be looking for European red mite and twospotted spider mites populations to increase on fruit crops in the weeks ahead. The above normal temperatures have helped mite populations increase rapidly on a variety of crops recently, especially woody ornamentals. (FH)

6. Pheromone Trap Catches

(FH)

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