Fruit Pest News

Volume 4, No. 17   July 21, 2003

An 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. Rhizopus Rot of Peach and a New Fungicide

    2. Bitter Rot of Apple

    3. Bitter Rot of Grape

    4. Vegetable Corner: Cucurbit Powdery Mildew

    5. Oriental Fruit Moth

    6. Pheromone Trap Catches and Biofixes

Reminder

For the remainder of the growing season, Fruit Pest News will be issued every two weeks. The weekly schedule will return at the beginning of the 2004 growing season.

1. Rhizopus Rot of Peach and a New Fungicide

Rhizopus rot is a common post-harvest rot of peach that can spread rapidly in containers of non-cooled fruit. It is characterized by a fluffy, black fungal growth on a rotted area of the fruit, accompanied by juice leakage. Before the mold is produced, Rhizopus rot can be detected by an easy slippage of the skin from the rotting flesh. Skin slippage does not readily occur over brown rot lesions.

Most brown rot fungicides used during the pre-harvest period are not effective against Rhizopus rot. The most effective is Elite, applied at the maximum labeled rate. Botran can be used, but has a 10-day preharvest interval and has no activity against brown rot.

Scholar 50W is a fludioxonil product from Syngenta that was recently registered for post-harvest use in stone fruit. Scholar has good activity against Rhizopus rot and brown rot infections that occur during and after the harvesting process. The labeled rate is 8 to 16 oz per 200,000 pounds of fruit. Scholar can be applied as a dip or through spray nozzles in the packing line. In the latter, the spray volume is 25 to 100 gallons, or if using low-volume systems, 7 to 25 gallons. For dips, the fruit is dipped for 30 seconds and allowed to drain. The solution is replenished after 200,000 pounds of fruit.

Handling practices are as important for preventing Rhizopus rot as for brown rot. Avoid puncturing fruit skin, and store harvested fruit at less than 40 F. (SB)

2. Bitter Rot of Apple

Bitter rot is the most potentially explosive of the apple fruit rot diseases. We sometimes get reports of growers, especially homeowners, losing the majority of their fruit to this disease. The restrictions placed on the most effective group of fungicides, the EBDC's, have increased our problems with bitter rot. The disease can move so rapidly because infection can occur in as little as 5 hours of wetting at 79-82 F. At 80 F, lesions can develop and produce spores within 11 days of infection. Fruit remain susceptible right up until harvest.

Identification. Bitter rot is recognized by round, sunken lesions on the fruit. When very young fruit are infected, the lesions appear as tiny gray-brown flecks, which usually do not develop further until fruit begin to ripen. Fruit infections occurring a month after petal fall begin as small, round, slightly sunken areas that are light to dark brown in color. As the lesions enlarge, concentric rings of spores or their fruiting bodies form around the center. Decay lesions extend in a cone-shaped pattern toward the core, observed by cutting the apple in half through the decayed area. This V-shaped lesion distinguishes bitter rot from white rot, which has a cylindrical decayed area.

Cultural control. A feared disease, bitter rot is controllable if trees are well-pruned and air circulation is good in the orchard. The removal of excess growth improves drying conditions and penetration of sprays into the canopy. Try to prevent the buildup of inoculum by removing overwintered mummified fruit, if feasible, or even diseased fruit during the growing season. Leaving fire-blighted shoots on the trees can increase fruit rot problems, because the fungi can colonize them. Do not allow prunings to pile up near the orchard. Prunings can be left on the orchard floor if they are chopped with a rotary or flail mower, which removes the bark on which the fungus can reproduce.

Monitoring bitter rot. Orchards should be scouted regularly for this and other fruit rots. In orchards in which this disease has been a problem, or in highly susceptible varieties, inspect the trees each week, beginning in early June. Highly susceptible varieties include Golden Delicious, Fuji, Arkansas Black, Granny Smith, Empire, and Yellow Newton. Almost as susceptible are Gala, Stayman, Jonagold, Jonathan, Grimes Golden, Ginger Gold, and Braeburn. Among the least susceptible are Delicious, Rome Beauty, and Winesap, but no variety has a high level of resistance.

Inspect as many fruit as is practical (this exercise provides another case for keeping trees trained to a manageable height). If more than 3 infected fruit are observed on any tree, a change in the control program may be needed. The needed change may be in the tree training and other cultural practices mentioned above. Or it may be in the spray program. The spray interval may need to be shortened (especially if the weather is rainy), the fungicide choice may need to change (a bitter rot fungicide should be used in each cover spray), or the application method may need adjustments.

Chemical control. The need for fungicidal protection against bitter rot is greatest from midseason until harvest, although some fruit infection can occur as early as petal fall. Where bitter rot has been troublesome in the past, use an EBDC fungicide (Dithane, Manzate, Penncozeb, or Polyram) as long as the label allows (until 77 days before harvest). Tank mixing with captan after petal fall will improve control of white rot and black rot.

Ferbam is also very effective against bitter rot, and can be used until 7 days before harvest. However, ferbam produces a gray residue that is unsightly on fruit. Other disadvantages of ferbam are increased cracking on Stayman, and russeting on certain varieties if ferbam is applied in the pink through first cover sprays. Russeting is not a problem when ferbam is used after the first cover spray. Like the EBDC's, ferbam applied alone may not provide adequate control of white rot or black rot.

Captan, ziram, and thiram are good bitter rot materials, and should be used at full rate where problems with bitter rot are expected. The addition of a 1/2 rate of Topsin M helps with control of sooty blotch and flyspeck, as well as white rot and black rot. The new strobilurin fungicides, Flint and Sovran, appear to be slightly less effective at the rates commonly used. Cover sprays should be applied at 10- to 14-day intervals, using 10-day intervals during wet, rainy periods. (SB)

3. Bitter Rot of Grape

If you are seeing a rot of mature berries (8% sugar or above), the culprit is probably bitter rot. Black rot, which has a similar appearance, occurs in green berries. Bitter rot can be a particular problem to wine grapes because of the unpleasant flavor it can impart to the wine. Catawba is particularly susceptible.

The causal fungus usually invades a berry from the pedicel (berry stem). As the berry rots, concentric rings of fungal fruiting bodies appear. Within a couple of days, the berry softens and is easily detached. Berries that do not fall off continue to dry and become firmly attached. When shriveled, the berries look much like berries affected by black rot or other fruit rots.

The bitter rot fungus overwinters on fallen leaves and berries and in the bark of year-old canes. The disease cycle on fruit starts shortly after flowering, when the fungus invades the natural warts on the berry stems and remains latent until the berry reaches maturity. It then invades the stem and moves into the berry, where a rot forms and spores are produced on the surface. Spores that are splashed to injured fruit cause new infections.

Generally, broad-spectrum fungicides such as Benlate, captan, ferbam, and mancozeb are effective in controlling bitter rot. Used around bloom time and afterward, these materials help prevent pedicel infections. Late season sprays are needed to prevent secondary infections. (SB)

4. Vegetable Corner

Mid-summer signals the arrival of powdery mildew season for cucurbit crops. This disease is one of the few in which scouting is an effective means of determining when to begin spraying. The fungicides used routinely prior to powdery mildew season (chlorothalonil, maneb, or mancozeb) generally are not sufficiently effective against mildew for use during mildew season. Watch for the circular, white patches on leaves and stems of susceptible varieties. When they appear, add Nova, sulfur, or Procure to the basic fungicide. When using Quadris or Flint, the addition of a mildewcide is not necessary. However, Quadris and Flint should always be alternated with non-strobilurin fungicides.

Note: When using Flint or Quadris, do not alternate with the related fungicide Cabrio, for two reasons: 1) Cabrio is not effective against powdery mildew, and 2) all three materials are in the strobilurin class, which would make such a program a haven for resistance development in the fungi.

In selecting a powdery mildew material, keep in mind the plantback restrictions. Nova has a 30-day plantback interval before planting any crop at the treated site, whereas Procure has a one-year interval for most non-labeled crops. Leafy vegetables can be planted after 30 days, and root vegetables after 60 days following application. Procure is labeled for use on apples, pears, grapes, cherries, strawberries, and cucurbits, and there is no plantback restriction for these crops. (SB)

5. Oriental Fruit Moth (OFM)

While determining the biofix and calculating Degree Days can be helpful, it is important to continue with orchard monitoring. In Pennsylvania (Fruit Times Newsletter, Vol. 22, No. 11) they use the Skybit model to predict Oriental fruit moth (OFM) egg hatch. From in orchard egg counts, they determined that the egg hatch model on peaches is running about 7 days ahead of the real crop situation. Conversely, in apple orchards, the OFM are developing slower than that predicted by the model.

The timing of applications made using the biofix and Degree Day accumulations requires the grower to continue trapping. The prediction only tells the grower when the next generation is supposed to happen and not how much pest pressure will occur. The determination of whether to treat or not and how many sprays to apply depends on pest density and previous damage. In the case of the third generation OFM, the decision on whether to treat or not is dependent on the existence of pest damage from the second generation OFM and trap catches exceeding 5 moths/trap/week. In Nashville, treatment for third generation OFM was recommended at 2,200 DD and 2,300 DD after biofix for apples and (peaches, plums and nectarines), respectively. This was June 29 for apples and July 2 for peaches, plums and nectarines.

For apples, fourth generation OFM is difficult to predict late in the season because of overlapping of generations. Egg laying can occur from August through October. Continue to trap and apply an insecticide within 7 to 10 days of a pheromone trap threshold of 10 moths per trap per week. Maintain spray intervals on a 2 week schedule for as long as the threshold is exceeded. (FH)

Codling Moth on Apple
Third generation codling moth requires an insecticide at 2,250 DD after biofix if pheromone trap catches exceed 5 moths per trap per week after 1,900 DD after biofix and/or if fruit damage caused by second-generation larvae is observed. In orchards adjacent to abandoned orchards or where old bins are placed near the orchard, codling moth populations may be extremely high and trap catches may stay high between generations. If this is the case, additional insecticide applications may be necessary. Recommended codling moth insecticides should provide enough residual to support the use of 14 day spray intervals (FH)

Preharvest Intervals
Remember to always check the pesticide label and be aware of preharvest intervals, especially as harvest approaches. These intervals vary widely and are often different for apples and peaches. Some examples of apple insecticide preharvest intervals are zero days for Dipel; 14 days for Confirm; 21 days for Asana; 28 days for Avaunt and Dimethoate; and 45 days for Apollo. (FH)

6. 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