Fruit Pest News

Volume 3, No. 11    May 20, 2002

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. Current Conditions

    2. Grapes: Spray Program Strategies for Black Rot

    3. Grapes: Time for Botrytis Spray

    4. Apples: How to Deal With Fire Blight Now

    5. Tree Fruit: Pheromone Trap Catches

1. Current Conditions - - FROST??!!

Who would have thought it - - frost in late May in Tennessee? It happened yesterday and today and probably will tomorrow. We are in the midst of a 4-night period of near-record lows. The frost was light and patchy in most areas, and fruit crops probably will not be affected. However, frost-sensitive vegetable crops were damaged. The unusually cold weather is not supposed to last much longer, and rain is not in the forecast. An adjustment in your disease-control program is probably not needed. However, this weather is favorable for powdery mildew of grape and apple, and frost injury can create an angular leaf spot outbreak in strawberries if the causal bacterium is present in the field.

Last week's weather included two rainy days with up to 8 inches of rain falling in parts of West Tennessee on Friday. Cedar-apple rust had been light, until a recent infection period produced a heavy "dusting" of golden-orange lesions on apple leaves. The rust season should be coming to an end, as far as new infections. However, apple shoots remain susceptible to powdery mildew until terminal growth ceases. (SB)

2. 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. Sounds good, but wait. Notice in the table above that infection can occur due to dew; that is, at temperatures typical of springtime nights, black rot infection can occur in one night because of the moisture provided by dew. I monitored wet periods and temperature in a Sumner County vineyard one year, and found that infection periods occurred most nights. I compared two schedules in the trial: protectant vs. postinfection. With so many infection periods, the postinfection schedule saved only one fungicide application (8 for the postinfection, 9 for the protectant).

Another consideration is that control of Phomopsis cane and leaf blight and downy mildew with the postinfection schedule can be weak. Conditions required for infection by these two diseases can differ from those for black rot. Furthermore, there are no eradicative fungicides for these diseases.

While a protectant approach for black rot is preferred, the eradicative technique is needed at times. Black rot infections can occur if the weather becomes rainy some time after your last fungicide application (e.g. 7 days or more). In that case, a black rot fungicide with good eradicative activity should be applied as soon as possible. Nova, Elite, or Bayleton would be good choices. The strobilurins (Abound, Flint, Sovran) have some eradicative activity, but should be used primarily as protectants. (SB)

3. Time for Botrytis Spray in Grapes

Early bloom is the time for the first application of a Botrytis fungicide. Botrytis bunch rot can be a problem in tight-clustered French hybrid or Vinifera varieties. Two recently labeled fungicides should help us in our efforts to prevent bunch rot. Vangard 75WG and Elevate 50WG represent two new classes of chemistry, so they will help us in resistance management.

Vangard is used at 10 oz per acre at early bloom and at berry touch, veraison, or preharvest (20 oz per acre per year, 7-day PHI). See the label for precautions for use near aquatic areas. Elevate is used at 1 lb per acre at early bloom, bunch pre-close, and veraison, up to and including the day of harvest (up to 3 applications). Rovral 50WP may be applied at 1.5 to 2 lb per acre at early bloom, prior to bunch closing, veraison, and preharvest (7-day PHI). Although the Rovral label allows 4 applications, it is suggested that no more than 3 applications be made, in light of resistance problems that have occurred in other areas. Benlate can be used at 1 to 1.5 lb per acre at early bloom and repeated 14 days later. (SB)

4. How to Deal With Fire Blight Now

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)

5. Pheromone Trap Catches and Biofixes

Tufted apple budmoth (TABM) has been an important pest of apples in areas of the Southeast. Larvae feed on leaves and fruit (usually surface feeding). Second generation larvae can feed within the calyx end of the fruit. If you have a history of this pest, it is important to remove food sources from overwintering larvae by keeping the orchard floor clean of broadleaf weeds and new apple sucker growth, especially in the early spring before bloom.

I have considered TABM more of a North Carolina problem but we are starting to see this pest in Tennessee. This year, the pest was detected from pheromone traps in both Putnam and Bradley Counties. The biofixes for Putnam and Bradley Counties are April 19 and April 27, respectively. TABM has two generations with egg laying for the first generation of larvae occurring around June and that for a second generation in August or September.

The timing of insecticide applications is critical to the management of this pest. Degree Day calculations are determined by using a 45 F lower base, the same as used with Oriental fruit moth. See the Table on page 32 of the 2002 Integrated Orchard Management Guide for Commercial Apples in the Southeast to aid in calculating degree days from minimum and maximum daily temperatures. The cumulative number of degree days from the point of biofix is used to predict percentage egg hatch and to time insecticide applications against first generation larvae. If population densities of TABM are high enough to require two sprays per generation, the first application should be made at 10 percent egg hatch, which occurs between 800 and 900 Degree Days after biofix. A second application should be made 14 days after the first. Where populations only require one spray, make the application between 10 and 30 percent egg hatch of the first generation, or from 800 to 1,200 Degree Days after biofix.

The number of sprays made for second TABM are dependent on harvest date. For apple cultivars harvested by early September, a single application at 2,600 Degree Day after biofix (early to mid-August) will suffice. A second application in late August or early September should be made to cultivars harvested after mid September.

Nashville (Davidson County) Pheromone Trap Catches for 2002

Biofix for RBLR in Davidson County estimated as occurring on March 14

Bradley County Pheromone Trap Catches

Codling moth biofix was April 14

Putnam County Pheromone Trap Catches

Obion County Pheromone Trap Catches

(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