Fruit Pest News

Volume 4, No. 12   June 3, 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. Apple Scab: Threat is Real

    3. Effect of Rain on Fungicide Washoff

    4. Apple: Fire Blight Trauma Blight Reminder

    5. Apple: Managing Fire Blight-Affected Trees

    6. Strawberry: Disease Control after Harvest in Matted Row Plantings

    7. Apple: Redbanded Leafroller

    8. Tree Fruit: Pheromone Trap Catches and Biofixes

1. Current Conditions

Cool and wet continues. Temperatures remain 10-12 degrees below normal on most days, with frequent, light rains and cloudy weather. Expect problems with diseases such as scab and sooty blotch and flyspeck of apple, Phomopsis cane and leaf spot of grape, and common leaf spot of strawberry. (SB)

2. Apple Scab: Threat is Real

Most Tennessee apple growers consider scab to be a rather minor disease because our hot, dry summers generally limit the development of this disease. However, this is not a typical summer, at least not to this point. Weather conditions remain abnormally cool and wet. If the scab fungus is present in your orchard, it will proliferate under these conditions. How often you should spray under these conditions depends on many factors. The following article provides some guidelines regarding reapplication of fungicides during rainy periods.

Fungicide use should center on captan. Use the full labeled rate. You can tank mix the captan with an SI (Nova, Procure, or Rubigan) to obtain curative activity in the leaves, but captan is needed for good control of scab on the fruit, where the SI's are weak. Remember that we want to minimize the use of SI fungicides to delay the development of resistance to them. Use a full rate, if you do decide to use an SI (see the May 19 issue of Fruit Pest News). You can also use Flint or Sovran to obtain some curative activity in the leaves, but they are rather weak on fruit scab and are limited to 4 applications per year. Dodine (Syllit) is very effective against fruit scab if resistance to dodine has not developed in the population.

These weather conditions also favor sooty blotch and flyspeck. With that in mind, selection of captan, Flint, or Sovran for the scab sprays will serve to control these fungi also. The SI's and Syllit will not control sooty blotch and flyspeck. (SB)

3. Effect of Rain on Fungicide Wash-Off

If rain if 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.

4. Fire Blight: Trauma Blight Reminder

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, even if the leaves are still wet. Generally, the potential for severe trauma blight is greatest in orchards showing fire blight symptoms. However, it is possible to experience new fire blight symptoms after a trauma event even if no symptoms were present before the event, due to the presence of epiphytic bacteria. Other than the "hail spray," streptomycin should not be used after petal fall. The hail spray is pretty much a desperate act to save a crop, and streptomycin may have some value in protecting injured tissue. On non-injured tissue, however, streptomycin is not effective in preventing fire blight infection and the resulting shoot blight.

5. Fire Blight: Managing Affected Trees

Look at the new growth for browning and wilting of leaves, indicating the shoot blight phase of fire blight. Efforts to limit secondary spread by cutting out fire blight strikes are most successful if the strikes are removed immediately after they appear. Make the cut 10 to 12 inches below the blighted area. Sterilizing the pruning tool between each cut is no longer considered to be necessary. 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 19 issue of Fruit Pest News. 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. 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. (SB)

6. 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.

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)

7. Redbanded Leafroller

The redbanded leafroller was caught in the pheromone trap as soon as it was put out so I estimated the biofix as March 8. The first generation redbanded leafroller caterpillars have matured, pupated and become moths with pheromone trap catches possibly peaking. Trap catches have been on the rise since mid-May. While redbanded leafrollers prefer to feed on apple, they can feed on a variety of plants. Some of the other fruit that can be attacked includes plums, grapes, peaches, raspberries, cherries and strawberries. The damage to the foliage (webbing and folding of leaves together) is seldom a serious problem (only during extremely high infestations). The damage to the fruit is the main concern. The first generation larvae fed on the small fruit and made shallow, irregularly shaped cavities in the fruit. The damage is often where two or more fruit touch and while some fruit will drop, others will develop thick corky tissue resulting in misshapen culls. This next second generation of larvae and the third generation will feed shallowly on the skin where a leaf is webbed to the fruit or where apples occur in a cluster. This damage is an entryway for fruit rot infections. The injured fruit also do not store well because of moisture loss. In heavy infestations, the fruit tend to drop before the normal harvest time. The next cover spray or two will be important in controlling this pest. (FH)

8. Pheromone Trap Catches and Biofixes

Nashville (Davidson County) Pheromone Trap Catches for 2003

* Biofix for RBLR in Davidson County estimated as occurring on March 8

Bradley County Pheromone Trap Catches

*Biofix for redbanded leafroller in Bradley County estimated as occurring on March 8
**Biofix for codling moth in Bradley County was April 5 (2 caught)

Putnam County Pheromone Trap Catches

*Biofix for Putnam County estimated as occurring on March 8

Obion County Pheromone Trap Catches

*Biofix for RBLR occurred prior to trap placement, estimated as occurring on March 8
(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, 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