Fruit Pest News

Volume 4, No. 10   May 19, 2003

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. Grapes: Spray Program Strategies for Black Rot

    2. Apples: Summer Rots Can Start Now

    3. Apples: How to Deal with Fire Blight Now

    4. Apples: Don't Use Low Rates of Sterol Inhibitors

    5. Tree Fruit: Oriental Fruit Moth

    6. Grapes: Grape Berry Moth

    7. Tree Fruit: Pheromone Trap Catches and Biofixes

Editor's Note: We apologize for the technical difficulties that prevented the links in recent issues of Fruit Pest News from operating. Those problems have been corrected and the links are now functional.

1. Spray Program Strategies for Black Rot of Grape

Black rot is the most destructive disease affecting grapes in Tennessee. In some of the literature and on some fungicide labels, you will see two types of strategies for applying fungicides for black rot control. One is referred to as protective or preventive. It involves spraying on a set schedule such as every 7 to 14 days. The spray interval chosen by the grower would be determined by the amount of rain: More frequent sprays would be needed in rainy weather, because most disease organisms are favored by wet conditions, and because rainfall can remove some of the fungicide residue.

The other strategy for applying black rot fungicides is referred to as a postinfection or eradicative schedule. With this method, certain systemic fungicides such as Bayleton or Nova are applied within 72 hours after the beginning of an infection period. An infection period occurs when moisture is present long enough at a certain temperature for the fungus spores to germinate and enter the plant tissue. The table below shows how long the tissue must remain wet at each of several temperatures for infection to take place.

The idea behind the postinfection method is to attempt to reduce the number of fungicide applications and to make the applications only when they are truly needed. Research is currently being conducted to determine whether the number of fungicide applications can actually be reduced when using this method in the South, where frequent heavy dews can cause numerous infection periods. The research also hopes to determine what impact such a program would have on other grape diseases.

A need for the eradicative technique is obvious at times, and weather monitoring equipment is not needed. Extended rainy weather, causing infections and preventing spray applications, would create an obvious need for a postinfection application of a fungicide, regardless of the type of program the grower is on. Use a fungicide with good eradicative activity, such as Nova, Elite, or Bayleton, if black rot infection is expected to have occurred. These fungicides are locally systemic, i.e. they are absorbed into the tissue onto which they are applied. Significant infection should not occur within 7-10 days of the last application of one of these systemic materials, because of their rainfastness. However, if only a protectant (mancozeb, ferbam, ziram, captan) was used, the residue can be completely removed after two inches of rain. As you can see, systemic fungicides have benefits both prior to and after rainy periods.

A word about the strobilurins (Abound, Flint, and Sovran): These broad-spectrum fungicides are locally systemic, just as Nova, Elite, and Bayleton are. However, the strobilurins do not have as good eradicative activity as Nova, Elite, and Bayleton, and should be used primarily as protectants -- rainfast protectants, but protectants nonetheless. (SB)

2. Apples: Summer Rots Can Start Now

The various summer rots are always a problem in the South. Some of these rots get started around this time of year, although the infections may not become apparent until mid- to late-summer. The recent deluges and humid weather can be expected to increase our problems with the summer rots if we don't keep our guard up.

White rot, in particular, is a fruit rot that can have a delayed effect. While lesions usually don't appear on the fruit until mid-summer or later, they can be the result of infections that occurred in May, and remained dormant until the sugar levels began to increase in the ripening fruit. The white rot fungus lives within the tree in dead wood. It can either invade already-dead wood such as fire blight strikes, or it can infect healthy wood through lenticels in the bark. In stressed trees, the latter type of infection can expand and produce cankers capable of killing branches. Spores of the fungus are spread to the fruit by rain.

Where problems with white rot are expected, captan and either Benlate or Topsin M should be included in the cover sprays. The strobilurin fungicides Flint and Sovran also appear to provide good control. (SB)

3. How to Deal With Fire Blight Now

Remember, do not use streptomycin to prevent shoot blight. It is ineffective and the unnecessary use of it can lead to resistance developing in the fire blight bacterium. An exception can be made by using it after a hail storm.

Should you cut out infected shoots? This has been debated for some time because of inconsistent results. The operation should be conducted as soon as possible after the fire blight strikes appear. If it is not possible to complete this task within a couple of days, it is probably best to let nature take its course. Remember that the job will need to be repeated as new symptoms appear.

The ugly stub method. The late Dr. Paul Steiner claimed that the old method of making a cut 10 to 12 inches below the symptom was useless, because the bacterium is usually systemic much further down the limb than that. It does no harm there, because of the natural resistance of the tree. Making a cut, however, breaches this natural defense, and a small canker almost inevitably forms around the cut. By making two cuts, the canker and its associated bacteria can be eliminated. The first cut is in 2-year-old wood, 2 to 4 inches above the supporting limb. A canker may form around the cut surface of the stub, but the second cut removes the stub at the supporting limb. This is done during the dormant season, when the bacteria are not active (paint or flag the stubs to make them easier to find).

Alternatively, you could wait until the dormant season and make one cut, and save a lot of work. But the ugly stub method allows those who wish to prune out fire blight during the growing season a method of doing so without leaving cankers around the cuts. (SB)

4. Apples: Don't Use Low Rates of Sterol Inhibitors

The sterol inhibiting fungicides Nova, Procure, and Rubigan are ineffective when used at half rates. This from the research program at Pennsylvania State University's Fruit Research and Extension Center, at Biglerville, PA. Some apple growers use the sterol inhibitor (SI) fungicides at half rates when tank mixing them with protectants such as captan, mancozeb, thiram, or ziram. The research projects indicated that the SI's failed completely in disease control when applied at lower rates. Protectant fungicides, on the other hand, decreased in effectiveness in direct proportion to the reduction in rate.

That makes two reasons not to use reduced rates of SI fungicides, the other being that this practice encourages the development of resistance in the pathogens. Apple scab populations resistant to the SI's are already appearing in New York. We must try to prevent this from happening here by using full rates of the SI's, always tank-mixing them with protectant fungicides, and not over-using the SI's. (SB)

5. Oriental Fruit Moth (OFM)

The biofix in Davidson County was on April 4, 2003. The accumulation of 500 DD occurred a month later on May 4 while 600DD occurred a little over one month later on May 8. In low OFM density peach, nectarine or plum orchards (less than 0.5 percent of the fruit damaged the previous year and pheromone trap catches that do not exceed 10 moths/trap/week anytime before the accumulation of 500 DD after biofix) a single insecticide application can be applied at 600 DD after biofix (May 8). In moderate to high-density OFM orchards, two insecticide applications should be made 14 days apart, the first at 500 DD after biofix (May 4 and May 18 applications).

6. Grape Berry Moth (GBM)

We put out a GBM trap here at the Ellington Agricultural Center in Nashville. There are a few wild grapes but no planted grapes. I have caught many small tan moths but only a few that I think might be GBM. I checked with Dr. Donn Johnson (Entomology Professor at the University of Arkansas) about my poor trap catches. They often catch another type of moth in the GBM traps that does not have the same dark markings as the GBM. It is different than the even lighter tan one that I have been catching.

In Arkansas, they have been catching GBM since April 18. Hatch can occur from 300 to 750 DD. They usually catch first flight moths only in traps placed along woods adjacent to the vineyard. However, this year they are catching GBM inside the vineyard at one of their trapping locations. They normally move the pheromone traps to the vineyard center in mid-May just before the summer flight of moths. Be expecting this summer flight soon. (FH)

7. 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

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