Fruit Pest News

Volume 3, No. 6    April 15, 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. Apples: Fire Blight Alert

    3. Blackberries: Time to Watch for Rosette Disease

    4. Strawberry Diseases

    5. Tree Fruit: Pheromone Trap Catches and Biofixes

1. Current Conditions

After a couple of days of cloudiness and showers, the weather has turned very warm. Daily highs in the low 80's are expected through Friday, with lows in the upper 50's to low 60's. This weather coincides with peak bloom in many apples, strawberries, and blueberries. Pollination should benefit from this weather, but so may some diseases. The articles below address some of these concerns. (SB)

2. Fire Blight Alert

Many apple varieties are in full bloom, so this week's weather should spell trouble for fire blight-susceptible varieties. Today's warm weather is expected to last through Friday. When warm weather lasts for more than 3 days during apple bloom, it almost always means fire blight infection. Such warm weather allows for rapid multiplication of the bacteria and movement to the blossoms on insects, so that they can quickly rebound from low levels. According to the MaryBlyt program, a repeat application of streptomycin may be needed in only 2 days in continued warm weather. All that is needed is moisture, and a simple dew will provide that.  Extended rainy periods are not needed for fire blight blossom blight to occur. A small shower or a dew that is heavy enough to wet the blossom will allow the bacteria that are in the blossom to enter the nectary and cause disease.

Since wetting events are usually the only missing element as far as infection is concerned, it is important that the operator pay close attention to dews. Do not count light dews. How heavy the dew must be to count it is a judgement call. Remember that the dew is heavier on the grass than it is in the trees.

It would be advisable to avoid any spray applications other than streptomycin to fire blight susceptible blocks in bloom this week. Such spray applications provide the wetting event needed for infection when other conditions are suitable. An extended warm period coinciding with full bloom is very suitable. If such sprays are needed on varieties that are susceptible to fire blight, consider tank-mixing with streptomycin. (SB)

3. Time for Blackberry Growers to Watch for Rosette Disease

Blackberries are preparing to bloom already, with blossom buds showing. Growers should look for evidence of rosette, also known as double blossom. This fungal disease can be very destructive, and indeed has been known to drive growers out of the blackberry business. The fungus can drastically reduce yields by causing sterility of the blossoms and through its debilitating effect on the plant.

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.

Fungicide applications will help to control infections that result from spores that enter the planting from nearby wild blackberries. Benlate alternated with copper fungicides can be applied at 10-14 day intervals, beginning about 6 weeks before harvest. Do not expect the applications to control rosette in this year's crop. Fungicides applied this year control rosette in next year's crop by protecting the new primocanes. Any rosettes you see this year arose from infections that occurred on last year's primocanes.

Fungicidal control also has a place if you do not currently have rosette, but are growing a susceptible variety. Shawnee is particularly susceptible. Cherokee, Comanche, Triple Crown, and Black Satin are also quite susceptible. Chester, Hull, Lock Ness, Arapaho, and Navaho have good tolerance. (SB)

4. Strawberry Diseases

Many strawberry fields are in full bloom during the current warm, humid period. Showers are predicted for Wednesday. Botrytis is favored by the humid, wet conditions. The warm day and night temperatures mean a short life span for blossoms, as each one should progress quickly opening to petal fall. What this means is that the window of opportunity for protecting that blossom against Botrytis is short. Most Botrytis infections occur during the bloom period, on open blossoms. It is during this period that fungicide applications do the most good. With this in mind, it may be advisable to shorten the fungicide spray interval from 7 days to 5 days until the weather moderates.

It would also be advisable to include an anthracnose material in the program, since this disease is favored by the high temperatures we are now seeing. Alternating Switch with an Elevate/captan tank mix would cover all the bases. Reserve the allowed 4 applications of Quadris for the harvest period, in case anthracnose appears at that time. (SB)

5. Tree Fruit: Pheromone Trap Catches and Biofixes

Redbanded leafrollers have been active since traps have been put out in both Nashville and Bradley County. I am putting the biofix at March 14 although it may even be a bit earlier. I expect that the biofix for Putnam County would be closer to mid-March if the traps had been out earlier.

Oriental fruit moths have been caught in Bradley County since the traps were put out (first catch of 6 on March 15) so the biofix is March 15. The biofix in Nashville was on March 28.

I have caught a single codling moth on four occasions since March 26 in Nashville. Since I have not caught two or more at a time, I have not yet set the biofix for Nashville. The Putnam County traps caught two codling moths on April 2 and four on April 3. This would put the biofix for the Putnam County site at April 2.

I have not caught any obliquebanded leafrollers in the Nashville trap.

Pheromone trap catches for Oriental fruit moth (OFM), redbanded leafroller (RBLR), obliquebanded leafroller (OBLR), codling moth (CM) and tufted apple budmoth (TABM) displayed below.

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

Putnam County Pheromone Trap Catches

Obion County 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, 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