Fruit Pest News

Volume 4, No. 4

April 7, 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. Grape: Early-Season Disease Control

    2. Blueberry: Twig Blight

    3. Apple: MaryBlyt Update

    4. Blackberry: Time to Watch for Rosette Disease
    5. Tree Fruit: Insect Pest Update

1. Early-Season Grape Disease Control

Grapes have begun growth and many are at the 1-inch bud stage (and growing rapidly) at which the first black rot spray is recommended. Other diseases such as Phomopsis cane and leaf spot and powdery mildew can also begin activity at this stage of growth. Choose a fungicide(s) based on the history of disease problems in the vineyard, using an efficacy table such as the one on page 17 in the small fruit spray guide, PB1197. Sterol inhibitor fungicides can be used in the early sprays, but keep in mind that they do not provide control of Phomopsis or downy mildew. The multi-site inhibitors (e.g. captan, mancozeb, maneb, ziram) are better choices for control of Phomopsis and black rot. The sterol inhibitors can be tank mixed with the multi-site inhibitors to provide control of powdery mildew and systemic control of black rot. Strobilurins have the broadest spectrum of activity, but are more expensive and are limited to 4 applications per year.

Considering the arsenal of fungicides available to the grape grower, it is a good idea to become familiar with the classification of the products, as listed on page 48 of the spray guide. (SB)

2. Twig Blight of Blueberry

Blueberry growers should look for twig blight, caused by Phomopsis vaccinii, beginning in early spring, since the symptoms can begin to show shortly after budswell. Control measures can then be initiated.

Individual buds turn brown and die, followed by browning of the bark around the bud as the fungus moves from the blighted bud into the twig. The infection usually advances inside the twig until most or all of the flower buds on a twig are killed. Click here http://www.ces.ncsu.edu/depts/pp/notes/Fruit/fdin010/fdin010.htm to see images of these symptoms. Click the images to enlarge them. The dieback does not progress into the older wood, making twig blight easy to distinguish from stem blight, caused by Botryosphaeria dothidea.

It is helpful to prune out and destroy the infected twigs, but this should be done prior to the bloom period, for maximum benefit. Many spores are released from infected twigs beginning shortly after bud break. Those that land in opening buds germinate and invade the twig. However, all is not lost if you have not done this. Removing infected twigs between this time of year and August is helpful in that it reduces twig blight in next year's crop. That is because dead twigs continue to produce spores, which land on and infect buds that become next year's flowers.

Preventive fungicide applications may be needed in plantings in which twig blight has been a problem in the past. Benlate plus captan is the most effective. If Benlate cannot be obtained, captan used alone, ziram, or the newly-registered strobilurin fungicide Cabrio are also effective.

The highbush varieties Murphy and Harrison are highly susceptible to twig blight and will surely need a control program. Croatan is moderately susceptible, while Reveille, Cape Fear, Bluechip, and Wolcott are relatively resistant. Some rabbiteye varieties are susceptible, especially Delite. (SB)

3. MaryBlyt Update

The MaryBlyt program for fire blight prediction indicates infection when 4 conditions occur: (i) any blooms are open; (ii) rain or dew occurs; (iii) the average temp for the day is 60 or above; and (iv) there have been enough accumulated heat units to allow colonization of the blossoms by the bacteria. The risk of infection is rated as "low," "medium," "high," or "infection." These ratings correspond to any one, two, three, or four of the previously-mentioned conditions, respectively. All the program needs from the operator is the high and low temperature for each day beginning at green tip, and rainfall or dew for each day beginning at first open bloom.

MaryBlyt guides your streptomycin spraying decision-making with this rule: You should spray if the risk of an infection is high andan infection is predicted for the next day.You use the weather forecast with the prediction function of MaryBlyt to determine whether an infection is predicted for the next day.

The Golden Delicious trees at the Plant and Pest Diagnostic Center began blooming yesterday, April 3. The cold weather of March 29-31 helped us by reducing the bacterial inoculum load. I think that it was the cold period around the beginning of apple bloom last spring that saved us from disaster. We had unusually hot weather for a week during bloom, usually a recipe for fire blight havoc. Cold weather prevents the fourth condition listed above from being positive. However, it does not require much warm weather to allow the population to build and spread to the open blossoms. (SB)

4. Time for Blackberry Growers to Watch for Rosette Disease

The key to managing rosette is to remove the infected blossom clusters before the blossoms open. That is because all of the spores that cause new infections come from infected blossoms. The infected blossom clusters should now be apparent. They are recognized as bunchy growths consisting of multiple shoots, rather than the normal single shoot. Such growths are known as rosettes or witch's brooms. The sepals (the green, leaf-like structures that enclose blossoms) are abnormally long and pointed. When the blossoms open, they will be pinkish and ruffled.

5. Tree Fruit: Insect Pest Update

We have been catching redbanded leafrollers in the Nashville (Davidson County) pheromone traps since they were put out on March 17. Trap catches peaked on from March 19 to 20 and are declining since the beginning of April.

The first Oriental fruit moth (OFM) was caught on April 2 followed by 2 on April 4, and 4 on April 7. Trap catches should built over the coming weeks. The OFM is a serious pest of peaches. It will attack the terminal growth early in the season and will attack the fruit in mid-summer. Other fruit trees attacked includes apple, pear, plum and cherry. The charcoal-color moth is smaller (1/4 inch) than codling moth (5/16 inch) and the redbanded leafroller and much smaller than the obliquebanded leafroller. Obliquebanded leafrollers and codling moths have not been caught yet.

Grape berry moth pupae overwinter in cocoons on the vinyard floor within debris and folded leaves. Adults begin to emerge before Concord grapes blossom. While it seems early, one moth was caught on April 4. Expect to see more of these as grape clusters form. The females will lay eggs on or near the grape clusters. First generation moths will occur in July and can be again detected by using the pheromone traps. (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