Fruit Pest News

Volume 4, No. 18   August 4, 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. Grape: Pierce's Disease

    2. Plum and Cherry: Black Knot

    3. Apple: Late-season Fungicide Selection

    4. Apple: Nectria Twig Blight

    5. Vegetable Corner

    6. Apple: Dogwood Borer Update

    7. Grape Berry Moth

    8. Tree Fruit: Pheromone Trap Catches and Biofixes

1. Pierce's Disease of Grape

Mid-summer is when Pierce's disease symptoms should begin appearing in grapes. Please watch for it and let us know if you suspect it to be present in your vineyard. We can give you instructions for confirming the disease. Confirmations are needed for your information and to help us track the distribution of the disease. Below is some information on this disease.

A hot-climate disease, PD is creeping northward as our winters continue to be milder than normal. Last winter's weather may have slowed its advance. PD is the most formidable obstacle to growing Vinifera grapes in the southern states. Other types of grapes are affected to a lesser degree.

PD is caused by the bacterium Xylella fastidiosa and is spread by certain kinds of leafhoppers. One particularly effective vector, the glassy-winged sharpshooter, has appeared in California and is causing a wine industry-threatening outbreak of PD in that state. X. fastidiosa can infect other types of plants. Many strains have been discovered. Some of these cause diseases of oak, sycamore, and maple (bacterial leaf scorch); plum (scald); peach (phony peach); and alfalfa (alfalfa dwarf).

The severity of PD appears to depend on climate. The bacterium appears to be sensitive to cold winter temperatures, and produces milder symptoms at higher altitudes, farther inland from ocean influences, and at more northern latitudes, even where vectors are plentiful. Please note that the vineyard in southeastern Tennessee had significant damage from PD, and many of the infected vines died.

PD symptoms begin as a drying or "scorching" of leaves http://csatwork.ucdavis.edu/Archives/Graphics/21600 srgb.jpg. The leaves become slightly yellowed along the margins before drying, or the outer leaf may dry suddenly while still green. Typically, the leaf dries progressively over a period of days to weeks, leaving a series of concentric zones of discolored and dead tissue.  About mid-growing season, when foliar scorching begins, some or all of the fruit clusters may wilt and dry up. Leaves drop, but petioles (leaf stems) remain attached to canes for an extended period of time. The bark on infected canes often matures in irregular patches, resulting in "islands" of green surrounded by brown, or the reverse.

Control of PD consists of vector control and roguing (removing) infected plants. More information is needed on where PD now occurs in Tennessee, and where it is likely to occur in the future (and to what degree). (SB)

2. Black Knot of Plum and Cherry

Black knot, caused by the fungus Apiosporina morbosa, is a common and destructive disease of plum and cherry. Losses in commercial orchards are usually not extensive except where the disease has become established. Residential plantings of edible or ornamental plum or cherry are commonly plagued with black knot. When left uncontrolled, purple leaf plum trees sometimes have to be removed.

Black knot affects only woody parts of the tree, mostly twigs, and is characterized by elongated swellings. When first formed, knots are olive green and corky in texture, and may be covered by a mold. Later, they turn black and become hard and brittle.

The fungus, living in the knots, produces spores that are discharged to other twigs, where they germinate and produce new infections. Young growth is very susceptible to infection. However, older branches can also be infected, especially if the bark has been damaged. Most infections occur during bloom, petal fall, and shortly afterwards. Some additional infections occur throughout active shoot elongation. Swelling of the twigs does not occur until autumn or the following spring. The knots develop rapidly the second summer.

Prune out all knots, cutting the stem a few inches below the knot. All pruned material should be removed from the area and destroyed or buried. Chemical protection against black knot can be afforded with Benlate, captan, chlorothalonil, or Topsin M. Chlorothalonil cannot be used after shuck split. A fungicide spray may be needed throughout active shoot elongation. During the critical periods mentioned above, an application may be needed every week.

Among the most susceptible varieties of plum are Damson, Stanley, Bluefree, and Shropshire. Moderately susceptible are Methley, Milton, Fellenberg, Bradshaw, and Early Italian. Less susceptible are Formosa, Shiro, and Santa Rosa. President is highly resistant. (SB)

3. Late-season Fungicide Selection in Apples

Keep in mind the preharvest intervals (PHI) of the fungicides needed for disease control near and during harvest. Although Flint and Sovran have good residual activity (around 21 days) the PHI of Sovran (30 days) precludes its use near harvest. The 14-day PHI of Flint reduces the time it provides protection after harvest begins. Spraying near and during harvest may leave undesirable residues on the fruit, but is necessary where fruit rots have become a problem. Captan, thiram, and Topsin M have 0-day PHI's, and would be the products of choice in such cases. In choosing a late-season fungicide, also consider the restricted-entry interval if you have employees. (SB)

Late-Season Apple Fungicides

* The restricted-entry interval for Captan 80WDG is 24 hr.

4. Nectria Twig Blight of Apple

Nectria twig blight is caused by the fungus Nectria cinnabarina. Symptoms of this disease can be confused with the shoot blight phase of fire blight. With both diseases, scattered terminal shoots wilt and produce a typical shepherd's crook at the end of the affected shoot. Fire blight infection is a serious problem, whereas the Nectria twig blight rarely causes economic damage.

The best way to differentiate the two diseases is to check for characteristic symptoms of Nectria twig blight. N. cinnabarina usually infects twigs through apple fruit stems that were left in the tree during the previous year's harvest. Infections are more common following years when rapid temperature drops in late fall or early winter may have contributed to winter kill of the infected stems. The fungus moves from the infected fruit stem into the subtending node. Sometime during June of the following year, the fungus succeeds in girdling the stem at the node and the shoot beyond that node wilts and dies. Unlike fire blight infections, the Nectria infection rarely extends more than one or two inches beyond the node. The margins of Nectria cankers are usually more distinct than are margins of fire blight cankers at this time of year. Within several weeks (by mid-July), nodes infected by N. cinnabarina will develop orange sporodochia, a kind of spore-producing structure. The presence of orange sporodochia on nodes below wilted twigs further differentiates these infections from fire blight.

Nectria twig canker is most common on terminal-bearing cultivars such as Rome Beauty, but it also occurs occasionally on other varieties such as Fuji and Empire. Fungicide sprays are not effective for controlling Nectria twig blight. Dead twigs can be removed during summer pruning or during dormant pruning the following winter. Leaving infected twigs in the tree during summer and fall does not have any significant effect on spread of this disease because the disease is more limited by fall/winter weather conditions than by presence of inoculum. N. cinnabarina colonizes many species of trees and shrubs, so inoculum is available from many sources other than apple trees. (Source: Dr. David Rosenberger, Cornell University)

5. Vegetable Corner

We are seeing the usual amounts of southern blight on tomatoes, peppers, and potatoes. This soil-borne fungus, Sclerotium rolfsii, is favored by hot weather. Southern blight is characterized by a rapid wilt of the entire plant, followed by death. In some cases, a strong, white, fan-shaped mold can be seen on the stem at the ground line, where the fungus attacks the plant. Avoid pulling soil (which may contain the fungus) up around the plant by cultivation. At the end of the growing season, turn the soil with a turning plow one time, then conduct no more cultivation.  Do not use a rotary tiller. The southern blight fungus is sensitive to deep burying. Terraclor can be used at planting time.

Tomato spotted wilt virus has been almost non-existent this year, unlike last year. It appears that the theory of the effect of early spring rainfall amounts on TSWV severity has much merit. The more rain that occurs in early spring, the less TSWV that occurs in the summer. In dry springs, be prepared! Unfortunately, little can be done to control this thrips-transmitted virus. If you see it, let us know. Leaf veins are purple on the undersides, and the leaves in the upper part of the plant may be blighted. Blighting often begins as bronze to purple spots on leaves. Green fruit may show dark lesions or a sooty appearance. Ripe fruit show yellow rings. (SB)

6. Dogwood Borer Update

Apple trees on dwarfing rootstocks often have burr knots that are key egg laying sites for dogwood borers. Serious damage can occur when the larval feeding extends beyond the burr knot and into healthy tissue. This can cause a loss of vigor and yield. Over time, girdling and tree death can occur. Applicable recent research at the USDA Appalachian Fruit Research Station and Virginia Tech's Alson Smith Agricultural Research and Extension Center was reported in the July 28, 2003 edition of the West Virginia University Extension Service online newsletter entitled The Orchard Monitor. Spiral-wrap trunk guards on apples have a significantly higher level of dogwood borer infestation than trees without guards. These type trunk guards may promote the formation of burr knots and/or allow for greater survival of dogwood borers. In addition to the use of insecticide sprays, control options to reduce infestations include: 1) replacing plastic spiral-wrap trunk guards with looser fitting and more porous types, 2) planting trees deeper to expose less of the rootstock shank, 3) coating the trunk with white latex paint, 4) mounding soil around burr knots on the exposed rootstock up to but not including the graft union, and 5) maintaining good weed control around the trunk. (FH)

7. Grape Berry Moth

Another grape berry moth (GBM) trap was put at a Middle Tennessee commercial vineyard in Williamson County in July. A better location was needed because of the poor trap catches at the Davidson County location where there are currently no cultivated grapes. The codling moth trap at Davidson County is also ineffective and a better location will be utilized in 2004. (FH)

8. 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

Williamson County (Nolensville) Grape berry Moth Pheromone Trap Catch

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