Fruit Pest News

Volume 5, No. 6  April 20, 2004

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. Apple: Rust Diseases

    3. Peach Disease Control

    4. Grape: Designing an Early-Season Fungicide Spray Program

    5. Apple: Powdery Mildew

    6. Tree Fruit: Insecticide Sprays and Oriental Fruit Moth

    7. Tree Fruit: Pheromone Trap Catches

1. Current Conditions

Last week brought bizarre weather, with snow on Tuesday and 80's before week's end. Two to three inches of rain fell between Sunday and Tuesday. Apples are entering petal fall, which is a key period for quince rust infections. See the article below for additional information on the rust diseases of apple. The cool to cold temperatures during the wet period last week should minimize disease infections. The MaryBlyt model did not indicate any fire blight infections here, but conditions are right now for infection when moisture is present. Showers are expected this week. The grape black rot model indicated only slight risks of infection last week. (SB)

2. Apple: Rust Diseases

We are entering a period of peak vulnerability of apple trees to rust. This would be a good time for a review of the rust diseases of apple. The following is modified from an article written by Dr. David Rosenberger, Cornell University, for the Scaffolds newsletter.

Three different rust diseases caused by Gymnosporangium species occur on apples in this area. Quince rust (G. clavipes) infects fruit (Fig.1) but does not cause leaf lesions. Cedar apple rust (CAR), caused by G. juniperi-virginianae, infects both leaves (Fig.2) and fruit of susceptible cultivars, but leaf infections are far more common than fruit infections. Hawthorn rust caused by G. globosum only infects leaves and is generally less damaging than CAR and quince rust. Hawthorn rust affects apple, pear, and hawthorn plants. On infected leaves, the early symptoms of CAR and hawthorn rust are so similar that the two species cannot be easily differentiated. By mid-August, hawthorn rust and CAR can be differentiated based on the appearance of the fungal fruiting structures (aecia) that form on the bottom sides of infected leaves. http://web.utk.edu/~extepp/profiles/apple/diseases/cedarapplerust.htm

All three of the apple rust diseases require red cedar trees as an alternate host. For both CAR and hawthorn rust, the spores that infect apples are produced on the orange, jelly-like telial horns that are generated by cedar galls during wet spring weather (Fig.2). Quince rust does not produce a spherical gall on cedars. Instead, the cylindrical gall produced by quince rust on cedar twigs might be referred to as a swollen canker. The cankers can be easily located only when they are wet and are producing short, knobby, orange-red telial horns during spring (Fig.3). Unlike CAR, where individual galls produce spores for only one season, quince rust infections can survive and produce spores on cedars for many years.

Nearly all apple cultivars are susceptible to quince rust, although the degree of susceptibility varies. Many apple cultivars are resistant to either cedar apple rust or hawthorn rust, but very few are resistant to both CAR and hawthorn rust. An interesting side note: Rust-resistant cultivars can develop a leaf spot that resembles frogeye leaf spot (Fig.4). This results when rust spores germinate and kill a few leaf cells before fungal growth is interrupted by the incompatible host reaction of the resistant cultivar. The killed leaf cells are then invaded by other secondary pathogens such as Alternaria, Botryosphaeria, or Phomopsis.

Infection is promoted by frequent showers interspersed with cloudy weather. Intermittent wetting promotes production and dissemination of rust spores, allowing time for spores to be blown into orchards from the alternate red cedar hosts. Also, light rains are less effective than heavy rains for washing spores out of the air. The period of peak susceptibility for rust infections on fruit occurs between pink and first cover. Infection of apple leaves continues until the telial horns on cedar are depleted, usually around second cover.

The EBDC fungicides (mancozeb, Polyram) and the SI fungicides (Rubigan, Nova, Procure, Bayleton) are all effective for controlling rust diseases on apples. The SI fungicides provide at least 96 hr of post-infection activity against rust infections on leaves. The EBDC fungicides do not provide any post-infection activity, although they may arrest the infection process when applied during the first 12—18 hr after rains begin. Captan and Topsin M are ineffective against rust diseases.

The strobilurin fungicides (Sovran, Flint) have provided good control of rust diseases in some trials and moderate levels of control in other trials. The inconsistent performance of strobilurins against rust diseases may be attributable to differences in spray timing. Several tests at the Hudson Valley (New York) Lab have shown that strobilurins controlled rust diseases as well as 3 lb/A of mancozeb if the sprays were applied before the infection period. However, the strobilurins have much less post-infection activity than the SI fungicides. In a 2001 trial where control plots had 69% of Jerseymac fruit with quince rust, Nova provided 98% control of quince rust when it was applied at either 3 days or 7 days after the infection period, whereas Flint provided only 23% control at 3 days post-infection and no control after 7 days.

Eliminating red cedars near orchards has long been recommended as one approach for reducing the incidence of rust diseases on apples. The number of spores reaching an orchard declines rapidly as the distance to the nearest cedar trees is increased. However, eliminating nearby cedars rarely provides complete control of rust diseases in areas where cedar trees are common, because small numbers of spores can be blown to apples from distant cedars. Where all cedars are eliminated within several hundred yards of an orchard, outbreaks of apple rust diseases are likely only in years when light rains cause extended intermittent wetting periods.

3. Peach Disease Control

Many peach cultivars are now at the shuck split stage. Fungicide applications for control of peach scab should begin at the shuck split to shuck fall stage, about one week after petal fall. Do not wait until all the shucks have fallen to make this application. Scab pressure is greatest from shuck split through 2nd or 3rd cover because scab spore counts are high during this time and the cool, wet weather that occurs during this time favors infection. Continue on 10-day intervals until 40 days before harvest. Bravo cannot be used after the shuck fall stage. Captan, sulfur, Pristine, Flint, and Abound are other peach scab products. Pristine, Flint, and Abound are strobilurin materials and should be rotated with non-strobilurin products that have scab activity, such as captan or sulfur.

Shuck split is also the time to begin bacterial spot sprays on susceptible varieties. Mycoshield should be applied every 10 days until 3 weeks before harvest. (SB)

4. Designing an Early-Season Fungicide Spray Program for Grapes

Grapes are progressing slowly, but some cultivars are entering the prebloom stage. Choose a fungicide(s) based on the history of disease problems in the vineyard, using an efficacy table such as the one on page 26 in the small fruit spray guide, PB1197. Sterol inhibitor fungicides can be used in the early sprays, but keep in mind that they do not provide control of Phomopsis or downy mildew. The multi-site inhibitors (e.g. captan, mancozeb, maneb, ziram) are better choices for control of Phomopsis, and they also control black rot. The sterol inhibitors (Nova, Elite, or Bayleton) can be tank mixed with the multi-site inhibitors to provide control of powdery mildew and systemic control of black rot. Strobilurins (Abound, Flint, Sovran) have the broadest spectrum of activity, but are more expensive and are limited to 4 applications per year.

Mancozeb and maneb would provide maximum control of black rot if sprays could always be applied as planned. As we all know, however, the weather often prevents that from happening. A rainy period that occurs some time after your last application (e.g. 7 days or later), may allow black rot infections to occur. In that case, a black rot fungicide with good curative activity should be applied as soon as possible. The sterol inhibitors would be good choices. Remember to tank mix them with Phomopsis materials. The strobilurins have some curative activity, but should be used primarily as protectants.

For varieties very susceptible to powdery mildew, a fungicide for powdery mildew control may be needed in the early sprays. The sterol inhibitors and strobilurins are good for this purpose, but many will need assistance from tank mix fungicides for control of Phomopsis and black rot.

Considering the arsenal of fungicides available to the grape grower, it is a good idea to become familiar with the classification of the products, as listed on page 50 of the spray guide. (SB)

5. Powdery Mildew of Apple

The mild winter followed by a dry spring may produce some powdery mildew conditions. A mild winter allows for good survival of the mildew-infected buds, and dry weather aids conidial germination and fungal growth of powdery mildew fungi. Watch for shoots that appear to be growing out entirely white, especially on inner, shaded sprouts. These shoots are developing from buds that were infected with powdery mildew the previous season. Fungicide protection is needed until terminal buds are set. The best mildewcides are the sterol inhibitors (Nova, Rubigan, Procure) and the strobilurins (Sovran, Flint). (SB)

6. Insecticide Sprays and Oriental Fruit Moth

The apples are at late bloom/petal-fall now and peaches are at shuck split in Nashville. This is a critical period of time for protecting these newly developing fruit. On peaches, nectarines and plums, an insecticide such as Imidan 70 W, Guthion 50W or Guthion 2L is used at Petal Fall to 1% Shuck Split to target plant bugs, plum curculio, scale and to a lesser extent Oriental fruit moth (OFM). The next spray is at Shuck Split to 10% Shuck Off for the same pests. The majority of the OFM egg lay has not occurred. At the Nashville location, we have only accumulated 167.8 DD after biofix through April 16. On stone fruit for low OFM-density orchards, a single insecticide application can be applied for this pest at 600 DD after biofix. In moderate to high-density OFM orchards, two insecticide applications are applied 14 days apart, the first at 500 DD after biofix.

On apples, the Petal Fall spray is coming up. Recommended insecticides at Petal Fall for plum curculio control are Guthion 50WP, Imidan 70WP, Avaunt 30WG, Actara 25SDG, Calypso 4F and Sevin 50WP. Plum curculio control is very important at the Petal Fall and First Cover sprays. While OFM insecticides are listed at Petal Fall, the targeted spray for first generation OFM on apple is still to come since we have only accumulated 167.8 DD through April 16. On apples, first generation egg laying is usually low and only one insecticide application between 400 and 500 DD after biofix is necessary for OFM. Actara 25SDG is not recommended for OFM but the other insecticides listed for plum curculio can be used.

San Jose scale chemical control at Petal Fall includes Dimethoate 4EC/400, diazinon 50 WP, Guthion 50WP, Esteem 0.86EC and summer oil (cauthion: Diazinon or oil applied with captan or Captec may cause phytotoxicity). Other pest that may occur at Petal Fall are white apple leafhopper, spotted tentiform leafminer, rosy apple aphid and green fruitworm. Check the 2004 Integrated Orchard Management Guide for Commercial Apples in the Southeast for the complete list of recommended insecticides for these pests. (FH)

7. Pheromone Trap Catches

Nashville (Davidson County) Pheromone Trap Catches for 2004

* The three RBLR caught in trap left over from last year. New RBLR trap put out 3-4-04. Biofix for RBLR in Davidson County estimated as occurring on February 29.
**Biofix for OFM estimated to have occurred on March 26.
***Biofix for grape berry moth (GBM) estimated as occurring on 3-30.

Bradley County Pheromone Trap Catches

*Four RBLR moths caught in trap from lat year. Biofix for redbanded leafroller in Bradley County estimated as occurring on February 29.
**OFM biofix on March 19.

Putnam County Pheromone Trap Catches

*Biofix for RBLR in Putnam County estimated as occurring on February 29.
**Biofix for OFM estimated as occurring on April 7.

Smith County Pheromone Trap Catches (Two orchards, a set of traps at each site))

* Biofix for RBLR estimated as occurring on February 29.
**Biofix for OFM estimated as occurring on April 9.

(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