Fruit Pest News

Volume 7, No. 19  August 9, 2006

An online newsletter whose goal is to provide all interested persons with timely information

on diseases and insects of commercial fruit and vegetable crops 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: What to Do at this Time if Pierce's Disease is Found

    2. Plum and Cherry: Black Knot

    3. Apple: Late-Season Fungicide Selection

    4. Cucurbit Downy Mildew Update

    5. Cucumber Beetle Feeding on Cucurbit Fruit

    6. Pheromone Trap Catches and Biofixes

1. Grape: What to Do at this Time if Pierce's Disease is Found

When Pierce's disease (PD) is found in a vineyard, the affected canes should be removed so that the insect vectors will not spread the bacterium from them to healthy vines. How much of the vine should be removed is debatable. Certainly, all symptomatic tissue should go. On vines having multiple trunks, the trunk supporting the affected cane or cordon should be removed. Severely affected plants with chronic infections should be dug up or cut off even with the ground. This type of case would be a highly susceptible variety in the second year of infection (In vines infected the previous season, the basal part of the cane will show symptoms first). In California, the pruning recommendations are rather severe, even for moderately infected vines: "Pruning a few inches above the graft union of vines with moderate foliar symptoms (some canes on entire cordons without symptoms or no symptoms at the bases of most canes) may eliminate Pierce's disease and allow vigorous regrowth the following year, but symptoms will reappear in many (30-40%) or most of these severely pruned vines the second year." Texas recommendations are to remove all infected vines, regardless of severity. Pruning recommendations in California and Texas are influenced by the presence of the glassy-winged sharpshooter, which we do not have here in Tennessee. As you can see, the basic problem in determining where to make the cut is that the presence of the bacterium inside the vine does not always correspond with the symptoms. It is probably best to err on the side of caution, and remove some tissue that may appear healthy.

Alternate host control. These plants can harbor the PD bacterium and serve as a source of infection for the grapevines. Many types of plants are hosts for the PD bacterium, and it would be impossible to eliminate them from the vineyard area. Instead, keep the vineyard floor and surrounding area mowed and control weeds in the row. In choosing a site for a new vineyard, locate it so that it is not within 150 feet of woodlots and fence rows. (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 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 visible residues on the fruit, but is necessary where fruit rots have become a problem. Captan, thiram, Pristine 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

4. Cucurbit Downy Mildew Update

Given the increased virulence of cucurbit downy mildew, it is prudent to keep an eye on where this disease is occurring. Remember that it overwinters in southern Florida and moves north each year. This year, it was first reported in southern Florida cucurbit crops in February and has since jumped to southern Georgia, the middle-Atlantic seaboard, and the Great Lakes region. A distribution map for July 31 is available here. There reportedly is a case in Henderson County, in western North Carolina. Details are lacking at this time.  Please let me or your Extension agent know if you suspect you have downy mildew. We would need to get the word out (anonymously).

Don't let the hot, dry weather fool you. There are plenty of thunderstorms around, to keep humidity high, and one night of heavy dew is all that is needed for infection, if spores are present. Conditions are not very favorable for spore transport, but new reports of downy mildew continue to surface, indicating that some transport is occurring. Apply chlorothalonil or mancozeb every 7 to 10 days, for protection. Be prepared to add a more specialized fungicide such as Tanos or Previcur Flex if downy mildew occurs in your vicinity. You need to choose two specialized fungicides, because they need to be alternated, to discourage resistance development. (SB)

5. Cucumber Beetle Feeding on Cucurbit Fruit

I was reminded while reading Kentucky Pest News that this time of year, the fruit of cucurbits are susceptible to chewing by cucumber beetles. This produces scarring of the fruit which can particularly be a problem on pumpkins. Inspect fruit (especially the underside) from now until harvest for the first signs of feeding damage and treat then to prevent additional scarring. Also, check for squash bugs that can be found on fruit, vines and foliage. When treating for these pests, be aware of the preharvest intervals for the insecticide that you choose. (FH)

6. Pheromone Trap Catches and Biofixes

Nashville (Davidson County) Pheromone Trap Catches for 2006
Oriental fruit moth (OFM), redbanded leafroller (RBLR), obliquebanded leafroller (OBLR), codling moth (CM), grape berry moth (GBM), variegated leafroller (VLR), black cutworm (BCW), diamondback moth (DBM)

* Biofix for RBLR in Davidson County was March 2.
**Biofix for DBM in Davidson County was March 30.
***Biofix for OFM in Davidson County is estimated to be April 3.
****Biofix for GBM in Davidson County was April 7.
^Biofix for OBLR in Davidson County was April 24.


Bradley County Pheromone Trap Catches for 2006

*Biofix for OFM in Bradley County on March 29.
**Biofix for CM in Bradley County on April 13.


Putnam County Pheromone trap Catches for 2006

*Biofix for RBLR in Putnam County estimated as March 3 (trap not out early enough).
**Biofix for OFM in Putnam county March 31.
***Waiting to verify biofix for CM, catch probably too early.


Obion County Pheromone Trap Catches for 2006

*Biofix for RBLR in Obion County estimated to be on March 3 (trap not out early enough).
**Biofix for OFM in Obion County on April 3.
***Waiting to verify biofix for VLR in Obion County, catch probably too early.
****Waiting to verify biofix for CM in Obion County, catch probably too early.
^Biofix for OBLR in Obion County.

Additional Pheromone Trap Catches in Davidson County for 2006
Sod Webworms (SWW), Armyworm (AW), Squash Vine Borer (SVB), Grape Root Borer (GRB)

*Biofix for SWW in Davidson County for 2006.
**Biofix for AW in Davidson County for 2006.
***Biofix for GRB in Davidson County for 2006.

Knox County Pheromone Trap Catches for 2006
Tufted Apple Budmoth (TABM)

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, Professor and Extension Entomologist

fahale@utk.edu

Both authors available at:

615-832-6802

fax 615-781-2568

Plant and Pest Diagnostic Center

5201 Marchant Drive

Nashville, TN 37211

Copyright 2006 The University of Tennessee. All rights reserved. This document may be reproduced and distributed for nonprofit educational purposes provided that credit is given to University of Tennessee Extension.

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