Fruit Pest News

Volume 3, No. 9    May 6, 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. Current Conditions

    2. Strawberries: Twospotted Spider Mites

    3. Strawberry Diseases

    4. Apples: Apogee Growth Regulator

    5. Fire Blight: Secondary Blooms

    6. Tree Fruit: Pheromone Trap Catches and Biofixes

1. Current Conditions

Stormy weather has been the hallmark lately. Hail storms continue to make life miserable for some. The tree fruit crops look good, although apple fruit set appears to be light in some areas. Fire blight is surprisingly mild, considering the hot weather we had during the bloom period. The peach crop is good, although some fruit is not enlarging due to damage to the ovaries by the freeze of March 22, when the trees were at pink tip to early bloom. Spider mite damage is occurring in strawberries. The weather forecast for this week is for scattered storms with seasonable temperatures. (SB)

2. Twospotted Spider Mites on Strawberries

Twospotted spider mites can reproduce rapidly during hot weather. This is because it takes the mites a shorter period of time to mature and start laying their eggs as temperatures increase. Twospotted spider mite outbreaks tend to occur during the heat of the summer on many host plants including apple trees. Sometimes they can build up during the spring, especially during hot, dry periods. Early April tended to be dry and temperatures were in the mid to high 80's from April 15 to the 21st.
On plasticulture strawberries, the black plastic promotes heat buildup to maximize growth and fruiting. Unfortunately, the heat can also be conducive to a buildup of twospotted spider mites. Early scouting of the strawberry plants can be used to detect the first signs of twospotted spider mite activity. This means turning leaves over to look on the underside of the leaves. Examine closely using a hand lens for the mites, their milky white eggs and their webbing. Start your spray program while mite numbers are low. If you allow them to build up in number, control will be more difficult and the plants can be stressed. Even if you finally control the mite outbreak, the damage to the plants will have already occurred. During a warm spring, it would be good to check leaves at least once per week.

Chemical control options include Danitol 2.4EC, Kelthane 50WSP, Agri-Mek 0.15EC, and Brigade 10WSB. Adjust spray volume and nozzle placement to assure maximum coverage of tops and the undersides of leaves. Read label and repeat as needed. Do not use Agri-Mek within 21 days of a second application.  (FH)

3. Strawberry Diseases

There have been several cases of leaf wilting in some plasticulture plants. In some cases, only the outer leaves wilted while in others, all of the leaves wilted and the plant died. The cause was determined to be Botrytis stem rot. The fungus attacked the base of the petiole, causing the leaf to die. In a field that had both plastic-mulched and straw-mulched plants, Botrytis was seen only in the plastic-mulched plants. Losses have not been high.

Hail has plagued the state the last couple of weeks. I hope you have not incurred this fate. If light hail has affected your field, your berries may be more subject to fruit rots due to the injuries. Botrytis, Rhizopus, and miscellaneous secondary fungi may seize the opportunity to invade the fruit. In such cases, an application of a broad-spectrum fungicide such as captan, thiram, Topsin M, Quadris, or Switch (heed plantback restrictions) may prevent some infections.

As a whole, disease problems have been light this year. You're doing a good job of disease management! (SB)

4. Fire Blight: Secondary Blooms

Most apple varieties have finished blooming, with the exception of secondary (tag-on) blooms. The MARYBLYT program ceases to predict fire blight infection periods and the need for streptomycin sprays after all blooms are gone. We do not recommend the use of streptomycin after bloom because of its ineffectiveness against shoot blight infections, and because such use increases the risk of resistance developing. Beware, however, of tag-on blooms. Streptomycin sprays may be needed as long as they are present. In such warm weather, tag-on blooms can easily become infected and negate all your hard work in preventing infections during the bloom period! If only a few secondary blooms are present, it would be worth your while to remove them by hand, because they potentially represent the beginning of a fire blight epidemic!

5. Apogee Growth Regulator for Apples

Note: It is too late this growing season to use Apogee growth regulator, but you may be wondering whether you should use Apogee in future years. The following article was written by Dr. Bill Turechek for the Cornell University fruit newsletter, Scaffolds.  (SB)

Apogee is a growth regulator and works by "shutting down" the growth of a tree and, therefore, is used primarily to control overly vigorous trees and reduce the need for seasonal pruning (see the articles written by Jim Schupp in the first two issues of Scaffolds). Apogee has value in fire blight management because when trees stop growing, they become relatively resistant to new blight infections and further expansion of established infections is arrested. Thus, Apogee can significantly reduce secondary spread of fire blight (i.e., shoot blight infections) in orchards where streptomycin sprays failed to provide 100% control of blossom blight. Apogee has no effect on shoot growth or fire blight for at least 10 days after application, so it acts too slowly to be of value as a rescue treatment for orchards with blight symptoms. Apogee is ineffective for control of the blossom blight phase of the disease and is registered only for apples, not for pears.

In mature orchards where trees have already filled their spaces, the decision whether or not to use Apogee can be based on a combination of its potential value as a vegetative growth inhibitor and as a supplement to fire blight control. In young orchards where trees have not yet filled their spaces, the decision is much more complex. Using Apogee for fire blight control in young orchards will reduce vegetative growth. That, in turn, will decrease profitability of the orchard in succeeding years because it will increase the number of years required for trees to fill their spaces and for the orchard to reach the break-even point. Because of this, growers need to seriously consider whether the delay to reaching full production or the reduction in fruiting capacity outweighs the potential loss due to fire blight.

6. Pheromone Trap Catches and Biofixes

Nashville (Davidson County) Pheromone Trap Catches for 2002

Biofix for RBLR in Davidson County estimated as occurring on March 14

Bradley County Pheromone Trap Catches

Codling moth biofix was April 14

Putnam County Pheromone Trap Catches

Obion County Pheromone Trap Catches

(FH)

The Fruit Pest News URL is: https://webmail.utk.edu/redirect?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