Fruit Pest News

Volume 4, No. 15   June 24, 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. Vegetable Corner: Spray Program for Disease Control in Cucurbit Crops

    2. Cherry Leaf Spot

    3. Raspberry: Septoria Leaf Spot

    4. Peach: Split Pits

    5. General: Update on Methyl Bromide Critical-Use Exemptions

    6. Peach: Oriental Fruit Moth

    7. Tree Fruit: Pheromone Trap Catches

1. Cucurbit Crops: Spray Program for Disease Control

Your strategy for spraying your cucurbit crop (watermelons, pumpkins, etc.) for disease prevention depends on the stage of growth of the crop.  Elaborate spray programs are seldom needed on young crops. Following is the spray program I recommend:

Early season:

Use a primary fungicide (chlorothalonil, mancozeb, or maneb) every 7 to 14 days (more frequently in wet weather, less frequently in dry weather).  Begin the program at vine tip-over to early bloom (3 to 4 weeks after seeding).  Inspect the field for disease symptoms beginning at seedling emergence so that the spray program can be started sooner than planned, if needed.  Also, look for insects regularly. If microdochium blight appears in pumpkin or squash, Flint or Cabrio should be alternated with the primary fungicide.  Otherwise, there is little need to rotate the primary fungicides with other fungicides in the early season. (Chlorothalonil, mancozeb, and maneb are not subject to the development of resistance, so continued use is not a problem).

Mid-Late Season:

Scout for powdery mildew when this disease becomes a threat, around mid-summer.  When the first powdery mildew colonies (circular, white patches) are seen, add sulfur, Nova, or Procure to the tank with the primary fungicide (preferably chlorothalonil, when powdery mildew is present).  This tank mix can be alternated with Flint or Quadris, if desired.  Various diseases can occur in mid-late season, and the choice of fungicides should be determined by what diseases appear in the current year, or have occurred in the field in previous years.  The relative effectiveness of the labeled fungicides can be found in UT's Commercial Vegetable Disease, Insect and Weed Control publication. Air blast sprayers are needed when canopies become thick.  Apply sprays every 7 to 14 days, depending on rainfall.  Add a fixed copper to the tank mix if angular leaf spot or bacterial leaf spot appear. (SB)

2. Cherry Leaf Spot

Cherry leaf spot is a problem in wet weather wherever cherries are grown. Tart cherries are particularly susceptible. The disease first appears as small, purplish spots on the upper surface of the leaves. These spots eventually turn brown. Most spots are circular; however, when abundant, they often coalesce and form large, irregular dead patches. The spots later separate from healthy tissue and drop out, leaving a "shot-holed" appearance. Frequently, there are areas around the spots that remain green, giving the leaf a mottled appearance. Leaves turn yellow and fall off.

Fungicides used for cherry leaf spot are captan plus either Benlate or Topsin M in the early cover sprays. Sterol inhibitors used in the preharvest period for brown rot control are also effective against leaf spot. If there is wet weather after harvest, dodine or chlorothalonil may have to be used once or twice, beginning 2 to 3 weeks after harvest. (SB)

3. Raspberry: Septoria Leaf Spot

Leaf spot, caused by Sphaerulina rubi, is often referred to as Septoria leaf spot (the asexual stage of the fungus is a Septoria species). It is a highly destructive disease that affects only raspberries. It was responsible for the failure of the highly susceptible variety 'Bababerry' in the Southeast. One of our main varieties, Heritage, seems to be fairly resistant.

Symptoms include small, tan to brown lesions, beginning on the lowest leaves and spreading up the plant. On more mature leaves, the lesions may become gray or silver. Heavily infected leaves turn yellow, become necrotic, and fall from the plant. Defoliation can be severe. The fungus overwinters primarily on dead leaves.

Coworkers and I conducted trials in two growing seasons in the early 1990's, in a Purple Royalty planting at the Middle Tennessee Experiment Station. The results of these trials provided information for making control recommendations, and provided data that supported the registration of Nova fungicide for control of Septoria leaf spot.

The only fungicide in the trials that was labeled at the time and provided control was fixed copper (basic copper sulfate was used in these trials). As a result, we have recommended copper for control of this disease since 1993. Better control of Septoria leaf spot has been made possible with the registration of Nova in 2000 and Captan 80WDG in 2003. The Captan label does not include Septoria leaf spot, but good control is possible. Captan 80WDG is labeled at 2.5 lb per acre, applied at 10 to 14-day intervals, with a 3-day PHI and 72 hr REI.

Nova provides even better control, but should be alternated occasionally with captan or copper to avoid exceeding the 10 oz/acre/year limit on the Nova label. Use 1.25 to 2.5 oz/acre, depending on the amount of foliage present, and repeat every 10 to 14 days. Nova may be applied up to the day of harvest.

For best results, begin sprays before infections begin, which may be as early as April, and continue until fall. (SB)

4. Split Pits in Peach

I have received a report of split pit problems in peaches (thanks for the report). The excessive rainfall we have had is probably to blame for the increase in split pit. Remember that there are two types of split pit: the visible type and the shattered pit type. In the visible type, the pit splits at the stem end and the flesh and skin remain open at the stem. In the shattered pit type, the pit splits at the blossom end. The danger in this type of condition is that the fruit may appear normal and is sold, but the consumer cuts it open to reveal a shattered pit and gumming in the pit cavity. Some of these fruit will be deformed and can be culled or sold as "seconds." Some varieties, notably early-maturing ones, are more prone to split pit than others.

Thank you for reporting your problems. It's not that we're glad you have problems, it's just nice to let other growers know what is happening in the industry! (SB)

5. Update on Methyl Bromide Critical-Use Exemptions

Last year, several southeastern states, including Tennessee, submitted a group application to EPA for a two-year extension on the use of methyl bromide after the January 1, 2005 phaseout. The affected commodities were strawberries, strawberry nurseries, tomatoes, peppers, and cucurbit crops. The fate of that application will be determined by an international committee, probably in November. The latest news is that the EPA thinks that the international committee will only allow exemptions for one year (2005) unless additional applications are made for 2006, using revised guidelines. Plans are being made to develop those applications this summer. The effort will again be a regional one, based at North Carolina State University. (SB)

6. Peach: Oriental Fruit Moth (OFM)

At the Nashville location, the biofix for OFM was around March 8. Using degree-day (DD) accumulations, the first generation OFM for a moderate to high density OFM peach, nectarine, or plum orchard would have required treatment with two applications 14 days apart starting at 500 DD (April 17). The second generation for a moderate to high density OFM orchard would have required treatment with two applications 14 days apart starting at 1,400 DD (May 29). Third generation OFM require an application at 2,300 DD after biofix if OFM trap catches exceed 5 moths/trap/week (2,040 DD have accumulated through June 23 so if 30 DD occur for each of the next 9 days, 2,300 DD is estimated to occur on July 2). (FH)

Penn State has an excellent web site for fruit pests. At one location, they have digital images showing what the insects look like in the sticky traps. This site can be accessed at http://frec.cas.psu.edu/insect_id.htm.  (FH)

7. 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