Fruit Pest News
May 14, 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. Peach Disease Control
2. Twig Blight of Blueberry
3. Effects of Loss of Benlate on Fruit Production
1. PEACH DISEASE CONTROL
The diseases of concern at this time of year are scab, bacterial spot, and green fruit rot. The dry weather may allow you to stretch your disease-control spray intervals to 14 to 21 days, but keep an eye on certain indicators that may signal upcoming problems. You may need to shorten the spray interval or change to another control product.
Bacterial spot. Most diseases are favored by rainy weather, but some react more explosively to it than others. Bacterial spot can become epidemic quickly because of the ability of the bacteria to reproduce rapidly and because of their ability to exist epiphytically on the various surfaces of the tree. The number of overwintered shoot "blacktip cankers" is not necessarily a good indicator of the disease potential, because the epiphytic population represents an invisible source of infection. The level of bacterial spot in previous years is probably the best indicator of the disease potential in a block. On susceptible cultivars, successful control of bacterial spot requires a program involving dormant pruning of cankers and use of copper sprays, early-season sprays of copper or Mycoshield, and some help from Mother Nature (dry weather).
When using Mycoshield, consider that it does not weather well. Because of its short residual activity, Mycoshield is most effective when applied shortly before rain, before it has a chance to inactivate. Try to apply within 24 hours before a wetting event, but allow time for the spray to dry. Copper sprays can be used at this time of year, but leaf damage can occur. If you choose to use copper now, use low rates of liquid coppers such as Tenn-Cop 5E (4-6 fl oz/acre) or Copper-Count-N (4-6 fl oz/acre) in at least 100 gal water per acre.
Peach scab. As with most diseases, the risk of infection can be determined by the severity of the disease in previous years. With peach scab, disease potential can also be assessed by examining fruit-bearing shoots for the presence of lesions. Scab lesions on twigs are brown, circular to oval in shape, and best seen on the parts of the shoot that have not yet turned from green to brown. If scab potential is considered low, sulfur should be adequate for control. Remember that sulfur is not very rainfast. Scab control is not needed within 40 days of harvest, because that is the length of time needed for symptom development after infection.
Green fruit rot. This is a good indicator for brown rot, as both diseases are caused by the same fungus. Green fruit rot is not often seen, but the prevalence of frost-killed, clinging fruit this year could result in infected fruit. Scout these and thinned fruit on the ground for an indication of brown rot pressure. Insect- or wildlife-injured fruit is also susceptible to brown rot and serves as a good indicator. Adjust cover sprays or initiate pre-harvest brown rot sprays early when disease pressure is high. Use captan or Abound if infected fruit is present. These materials will also control scab. (SB)
2. TWIG BLIGHT OF BLUEBERRY
Scouting for twig blight (Phomopsis vaccinii) of blueberry should begin much earlier than now, but it is not too late to obtain some benefit from pruning out diseased twigs. Individual buds turn brown and die, followed by browning of the bark around the bud as the fungus moves from the blighted bud into the twig. The infection usually advances until most or all of the flower buds on a twig are killed. Click here to see images of these symptoms (Click the images to enlarge them). The dieback does not progress into the older wood, making twig blight easy to distinguish from stem blight, caused by Botryosphaeria dothidea.
It is helpful to prune out and destroy the infected twigs, but this should be done prior to the bloom period, for maximum benefit. Many spores are released from infected twigs beginning shortly after bud break. Those that land in opening buds germinate and invade the twig. However, all is not lost if you have not done this. Removing infected twigs between this time of year and August is helpful in that it reduces twig blight in next year's crop. That is because dead twigs continue to produce spores, which land on and infect buds that become next year's flowers.
Preventive fungicide applications may be needed in plantings in which twig blight has been a problem in the past. Recent research at Michigan State University indicates that a Benlate (1 lb/acre) plus captan (5 lb/acre) tank mix is the most effective treatment for control of twig blight. Ziram and Aliette, also labeled for blueberries, were not as effective. Benlate plus captan is also effective against fruit anthracnose. In selecting fungicides, keep in mind that Benlate is limited to 3 applications per year, and Ziram cannot be applied later than 3 weeks after full bloom. Where twig blight has been a problem, a spray program should be begun at green tip through early bloom. (SB)
3. EFFECTS OF LOSS OF BENLATE ON TENNESSEE FRUIT PRODUCTION
As reported previously in Fruit Pest News, Benlate fungicide has been voluntarily discontinued by the manufacturer, Dupont. Sales of the controversial fungicide will be phased out by the end of 2002. Benlate has been the cause of many lawsuits and cost the company more than $1 billion in litigation expenses. The company said the move was not a product recall, but a "voluntary business decision" based on financial and legal problems associated with Benlate.
What impact will this move have on fruit production in Tennessee? Fortunately, there is a closely related fungicide, Topsin M, which has a virtually identical disease control spectrum and use-pattern as Benlate. Unfortunately, Topsin M is not labeled for use on every crop for which Benlate was labeled. Topsin M can be used on apples, stone fruits, strawberries, and pecans. Topsin M is not labeled for use on caneberries (brambles), blueberries, grapes, and pears, as was Benlate.
As I see it, the largest voids left by the loss of Benlate will be for control of anthracnose of blackberry, Eutypa dieback of grape, pear diseases, Phomopsis twig blight and mummy berry of blueberry, and resistance management for Botrytis in grapes and brambles. For some of these voids, help is on the way. A new federal label for captan that includes brambles is expected this year, and brambles registration applications for two new fungicides have been submitted to EPA by the manufacturers. At least some of these materials will be effective against anthracnose of brambles (Rosette disease can be controlled with Bordeaux mixture, but some of the new materials may also provide help there). For blueberries, I know of two fungicides that are nearing registration that will help with control of mummy berry and twig blight.
For now, Benlate is still available, and can be used until supplies are depleted. (SB)
The Fruit Pest News URL is: http://web.utk.edu/~extepp/fpn/fpn.htm
Contacts:
Steve Bost, Professor and Extension Plant Pathologist
Frank Hale, Associate Professor and Extension Entomologist
Both authors available at:
615-832-6802
fax 615-781-2568
Plant and Pest Diagnostic Center
5201 Marchant Drive
Nashville, TN 37211