Fruit Pest News

Volume 3, No. 21    August 19, 2002

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.

Fruit Pest News will be produced every two weeks until the end of the growing season.

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. Grapes: Downy Mildew

    2. Apples: Phytophthora Crown Rot and Collar Rot

    3. Apples: Fruit Rot Diseases

    4. Tree Fruits: Oriental Fruit Moth and Codling Moth

    5. Tree Fruits: Pheromone Trap Catches and Biofixes

1. Grape Downy Mildew

Downy mildew often becomes very noticeable in vineyards in late summer and fall. This disease causes direct yield losses by affecting flower and fruit clusters. Indirect losses can result from premature defoliation of vines due to leaf infections. Be wary of premature defoliation of vines after harvest, because this predisposes the vines to winter injury.

Summer spay programs are needed even where a conscientious program was followed during the spring. You can't rest! Springtime symptoms of this disease often go unnoticed. Furthermore, spores of this fungus are capable of riding on air currents for some distance, so they can enter a vineyard during the summer.

Leaf symptoms can vary depending on the age of the leaf when infected, and the temperature. Leaf spots can be large and yellow, with indistinct margins, or small, angular, yellow to necrotic, and limited by the veins. In late summer and fall, most leaf infections appear as the latter. Older leaves become infected first, and the disease spreads towards the leaves at the ends of the canes. Leaves eventually become dry and brown and fall from the cane. Such defoliation reduces sugar accumulation in fruit and decreases hardiness of overwintering buds.

Shoot tips, tendrils, and blossom clusters may also be infected. Young berries are very susceptible, appearing grayish when infected and covered with a downy felt of fungus sporulation. Although berries become less susceptible as they mature, infection of the rachis (the main cluster stem) can spread into older berries. Berries infected at this time generally do not soften or show a downy growth; instead, they become brown and shatter easily.

The downy mildew fungus overwinters primarily in fallen leaves. Survival is best on the soil surface and is enhanced by wet winters. The overwintering spores germinate in water in the spring to produce a spore that disperses to plant tissue by splashing rain.

Maneb, mancozeb, Ridomil Gold Copper, and Ridomil Gold MZ are excellent downy mildew fungicides, but have 66-day preharvest intervals. Among the fugicides used during the summer on grapes, captan, Abound, fixed copper, and ziram have the best activity against downy mildew. A 10 to 14-day spray interval should be followed until the harvest restriction is reached. Susceptible varieties should be sprayed after harvest also, so as not to incur the vine-weakening effects of defoliation by downy mildew. (SB)

2. Phytophthora Crown Rot and Collar Rot of Apple

Crown rot, collar rot, and root rot in apple trees are caused by Phytophthora species (root rots can also be caused by other types of fungi). These three diseases can occur on one tree and can be caused by a single infection, but are given different names to designate the part of the tree affected. Collar rot occurs in the scion part of the tree. This disease has decreased due to changing variety preferences and to the practice of raising graft unions above the soil line at planting. Crown rot is a disease of the rootstock portion of the tree. It has become more important because of the increased use of susceptible rootstocks such as MM106 and M26. Phytophthora root rot is a disease of the root system away from the crown region. It may occur along with crown rot or may occur by itself.

In the spring, symptoms of Phytophthora include delayed bud break, leaf discoloration, and poor growth. Foliage is sparse, yellowish, and may develop an early purple discoloration in autumn. In dry weather, infected trees are the first to exhibit symptoms of water deficit.

The most obvious symptom is a partial or complete girdling of the trunk with reddish-brown, water-soaked areas of dead tissue at the base of roots where they attach to the trunk. The entire underground portion of the trunk is usually water-soaked and brown with the dead area extending upward to the graft union.

The fire blight bacterium can cause rootstock cankers on M26 and M9 that are almost indistinguishable from crown rot. Fire blight cankers form in July and August following early season blossom and/or shoot infections. Rapid decline of trees on these rootstocks in late summer may be caused by fire blight cankers below the graft union.

Avoid planting fruit trees in poorly drained locations, and do not allow depressions to form around the base of trees. Where drainage is questionable, use the less susceptible rootstocks such as seedling and M9. Moderately susceptible rootstocks are M7, M26, and MM111.

Ridomil and Aliette fungicides are helpful in control, but should not be solely relied upon. They should be used in conjunction with cultural practices. Because Phytophthora diseases occur sporadically, it is usually not economical to treat entire orchards on a regular, preventative basis. Rather, use them in problem areas. Also note that these fungicides seldom are effective in reviving trees once the crown has become infected and moderate symptoms of decline have appeared. Ridomil can be used in the fall and spring only, applied as a soil spray. Dipping the roots of nursery-grown trees into a solution of Aliette prior to planting may reduce, but not eliminate, Phytophthora inoculum on roots. Aliette can be used as a foliar spray to nonbearing trees. (SB)

Apple Fruit Rot Diseases

Fruit are entering their period of highest susceptibility to rot diseases. It may be worthwhile to review the major rot diseases of apple fruit, so that your spray program can be adjusted accordingly, if needed.

Black rot of fruit is characterized by dark, firm rots that are not sunken. As the lesions enlarge, they may produce a series of concentric rings alternating from black to brown. Fruit infection can occur early in the season, as soon as the bud scales begin to loosen. This early infection usually involves the sepal, and results in blossom-end rot later in the season. The black rot fungus also causes frogeye leaf spot and limb cankers.

White rot, sometimes called Bot rot, can be destructive on certain varieties such as Golden Delicious, on which small, tan, developing lesions are often surrounded by a red halo. When fruit rot under warm conditions, as they usually do, the rot is tan to light brown, soft, and watery. Under cooler conditions, rotted areas are usually firmer and deeper tan, and resemble black rot. White rot fruit lesions extend in a cylindrical pattern to the core of the fruit.

Bitter rot fruit infections are circular, sunken, and brown in color. When developing lesions reach about the size of a quarter, concentric rings of spores are produced around the center of the lesion. Under moist, humid conditions, the spore masses appear creamy to pink to orange in color. Decay lesions extend in a cone-shaped pattern toward the core, as opposed to the cylindrical shape of white rot lesions. Numerous infections can occur and losses can be extensive in warm, wet weather.

Apple fruit rots are managed by good pruning practices to improve spray coverage and drying time. Either remove the prunings from the orchard or mow them with a flail mower. (SB)

Oriental Fruit Moth and Codling Moth

The accumulation of degree days (DD) has really added up this summer. As of August 18, the DD for Oriental fruit moth (OFM) in Nashville is 3,875.5. We are well past the third generation where an insecticide is recommended at 2,200 DD after biofix if trap catches exceed 5 moths/trap/week after 1,900 DD after biofix and if there is fruit damage caused by the second generation. The DD accumulations are not as helpful this late in the season because of overlapping generations which make it difficult to predict when egg atch of the fourth generation OFM begins.. Since continuous egg laying can occur from August through October, continue to monitor your pheromone traps each week to determine pest activity and the potential for late season damage. The Orchard Monitor (West Virginia University Extension Service) recommends spraying apples within 7-10 days of exceeding a pheromone trap threshold of 10 moths/trap/week. Maintain spray intervals on a 2 week (complete) or 5-7 day (alternate-row-middle) schedule for as long as the threshold is exceeded.

An insecticide application is recommended for third-generation codling moth at 2,250 DD after biofix if trap catches exceed 5 moths/trap/week after 1,900 DD after biofix and/or if fruit damage caused by second generation larvae is observed. We are well past this in Nashville at 3,118.5 DD accumulation for codling moth. I am not sure if another generation of codling moth will occur or not. I would continue to monitor using the pheromone traps and treat only if another moth flight is detected. The 5 moths/trap/week threshold could be used. (FH)

Pheromone Trap Catches and Biofixes

                                            Nashville (Davidson County) Pheromone Trap Catches for 2002

                                                Biofix for RBLR in Davidson County estimated as 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: 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