Fruit Pest News

June 25, 2001

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. Brambles: Diseases of Fruit

    2. Raspberry: Septoria Leaf Spot

    3. Grape: Downy Mildew

    4. Apple: Potato Leafhopper

    5. Tree Fruit: Monitor Trees For European Red Mite and Twospotted Spider Mite

    6. Tree Fruit: Insect Trap Captures

1. DISEASES OF BRAMBLE FRUIT

Rainy conditions during blackberry and raspberry harvest may bring on diseases of the fruit, such as gray mold (Botrytis) and anthracnose. It may be necessary to spray with a labeled Botrytis fungicide such as Benlate or Rovral. Ronilan, which is in the same class as Rovral, is labeled only for raspberries. That use will not be allowed after September 30, 2001, due to a voluntary termination of that registration by the manufacturer. The labels of these 3 products allow 2 applications during the bloom period, followed by up to 3 additional applications through harvest. Benlate is also effective against bramble anthracnose. The Benlate label disallows the use of Benlate after a planting is opened to the public as a pick-your-own operation.

Gray mold is characterized by a berry rot that becomes covered with a gray to brown, dusty spore mass. Gray mold can also develop in storage after harvest, especially in raspberries. To avoid post-harvest problems with gray mold, try to prevent injuries to the berries, harvest in the cool of the morning, and refrigerate promptly.

Anthracnose is characterized by dull brown, scabby, flattened drupelets on blackberry fruit. To determine whether there is a need to apply Benlate sprays to protect blackberry fruit from anthracnose, look for lesions on the leaves. These lesions are small, purple spots that develop white centers. Spores produced by these spots can splash to the berries and produce the scabby berry condition. In sites with a history of anthracnose, a Benlate spray program should be begun during bloom. Removal of the fruiting canes after harvest helps to prevent carryover of inoculum from one year to the next. (SB)

2. SEPTORIA LEAF SPOT OF RASPBERRY

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.

Nova fungicide was recently registered for control of Septoria leaf spot. Coworkers and I provided the data that supported this labeling. We conducted tests in two growing seasons in the early 1990's, in a Purple Royalty planting at the Middle Tennessee Experiment Station. The table below provides the results for each product we tested. Rather than present the detailed data, I have summarized them in this table. If you would like a copy of the complete report, please let me know.

The only labeled product that provided acceptable 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.

Nova was included in the trials because registration seemed promising. The IR-4 Program was pursuing a brambles label for rust control. Fortunately, the IR-4 project came to fruition, and Nova was labeled in 2000. As you can see in the table, Nova provides very good control of Septoria leaf spot. The rate used was 2.0 oz/acre (the labeled rate is 1.25 to 2.5 oz/acre), applied every 10 to 14 days. For best results, begin sprays before infections begin, which may be as early as April, and continue until fall. Nova may be applied up to the day of harvest. Alternate to copper occasionally, to avoid exceeding the 10 oz/acre/year limit on the Nova label. (SB)

3. GRAPE DOWNY MILDEW

Downy mildew usually makes its presence known about this time of year and later. 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. Summer spay programs are needed even where a conscientious program was followed during the spring.

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. 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)

4. APPLE: POTATO LEAFHOPPER

Feeding by potato leafhopper on the younger leaves causes the vascular tissue to become clogged. The leaf edges curl, turn yellow and finally brown. This damage is called hopper burn. The potato leafhopper nymphs are pale green and run sideways on the leaf. This pest has been shown to facilitate fireblight.

The white apple leafhopper feeds more on the older foliage and causes stippling (tiny yellow feeding spots) on the leaf. White apple leafhoppers also deposit lots of dark fecal spots on leaves and fruit. The nymphs of white apple leafhoppers run backward or forward.

Look to control white apple leafhopper starting at Petal Fall with insecticides such as Sevin 50 WP, Provado 1.6F, Auvant 30 WG or Dimethoate 4EC/400. Potato leafhoppers can be a problem starting around second cover. The threshold level for second brood nymphs of both these pests (in late July to early August) is one nymph per leaf. Treatment at this time will eliminate the need to treat for adults immediately before harvest. Generally, low rates of insecticide will control leafhoppers.(FH)

5. TREE FRUIT: MONITOR TREES FOR EUROPEAN RED MITE AND TWOSPOTTED SPIDER MITE

On apple, the mite threshold levels before July are 7 mites per leaf (85 percent infested leaves) and 10 mites per leaf (90 percent infested leaves) during July and August. Remember, European red mites overwinter on the tree in the egg stage while twospotted spider mites overwinter in weeds and grass on the orchard floor and move into the trees during late spring and summer.

Look for the predaceous lady beetle Stethorous punctum. This tiny, black predator feeds on mites. If mite populations are near threhold level and S. punctum are present, recheck in 3 to 4 days to determine if biological control reduces mite populations.

In orchards with Alternaria blotch, a lower mite threshold of 2 to 3 mites per leaf (70 to 75 percent infested leaves) may be necessary to minimize stress to the tree.

On peach, plum and nectarine, use miticide as-needed. Peaches tolerate moderate mite populations with little injury. Treat if mites are numerous or for the presence of mites and the on-set of bronzing or webbing. Overcropping, drought and other stress factors justify lower treatment thresholds. Many areas of the state have had hard rainfall on an almost weekly basis. This type weather will often help regulate mite populations, especially for twospotted spider mites. Still, hot temperatures are conducive for rapid mite build-up, so start checking. (FH)

6. TREE FRUIT: INSECT TRAP CAPTURES

                                                        Obion Co. (traps put out April 9)

 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