May 2001 Newsletter
Lindsey du Toit and Debra Inglis, editors
WSU Mount Vernon NWREC
16650 State Rte 536, Mount Vernon, WA 98273-4768
360-848-6140 (tel), 360-848-6159 (fax)

WSU Vegetable Pathology Team Newsletter
 

Hello!

Welcome to the 2001 field season and the May newsletter of the Washington State University Vegetable Pathology Extension Team. This newsletter follows the team's May conference call in which team members discussed current vegetable diseases, diagnoses and control. We hope you will use this information in your own program newsletters and activities.

New Vegetable Pathology Team Website Now Being Posted

Many thanks to Babette Gundersen, Nancy Liggett, and team members for the work done so far on the team's new website, http://mtvernon.wsu.edu/path_team/vegpath_team.htm It is divided into six sections: Meet the Team, Team Newsletter, Vegetables in Washington, Vegetable Events, Vegetable Disease Photo Gallery, Diagnostic & Management Resources. We are in the process of improving and expanding the site, and we appreciate your patience during the process of posting it in all of its entirety. We hope that it will be a useful resource in your vegetable disease diagnoses and management activities.

Other Useful Vegetable Disease Websites

Additional information about vegetable diseases can be found on other websites:

Cornell Plant Pathology Vegetable Disease Web Page http://vegetablemdonline.ppath.cornell.edu/Home.htm

Oregon State University On-line Guide to Plant Disease Control http://plant-disease.orst.edu/

The Ohio State University Extension Vegetable Crops Team http://www.ag.ohio-state.edu/%7Evegnet/

IPM for Greenhouse Growing http://www.msue.msu.edu/ipm/greenhouseAlert.htm

Need to submit a plant disease sample to the WSU plant clinics?

Find out how at http://www.prosser.wsu.edu/Faculty/Bentley/Bentley.html
http://www.puyallup.wsu.edu/plantclinic/index.htm#Sample Submission Procedures

Vegetable Diseases/Issues and Places to Find Information

Is the new Legend tomato resistant to late blight? Dyvon Havens and Skagit Valley Master Gardeners have recently issued a report about 'Legend' - the new early determinate tomato variety that has large red fruits and is well adapted to the PNW. It has recently been introduced by Territorial Seed Company. Three years of tests have shown this variety to be highly resistant to two of the most common strains of late blight in the maritime Northwest. Research at WSU-Mount Vernon has shown that under high disease pressure in the Skagit Valley, 'Legend' may not be as resistant to late blight as it is in other areas. Although the foliage displayed a high level of resistance, the fruits were susceptible to infection during the 2000 growing conditions. 'Legend' will be tested again at Mount Vernon during 2001. The following tables summarizes the results from the 2000 Tomato Late Blight Evaluations at WSU-Mount Vernon.

¹ The higher the disease progress value, the more severe foliar late blight.

^2,3 Matt's Wild Cherry was the resistant check; Early Girl was the susceptible check.

⁴ Means within the same columns followed by the same letter are not significantly different.

In view of these results, blighting of fruits may occur on this variety in some areas and so gardeners are advised to inspect plants closely, especially as the fruits start to ripen. If fungicide applications are initiated, they need to be used preventively, i.e., before symptoms appear in order to be effective. For organic growers, several fixed copper fungicides are available. Fungicide labels specify a range of rates to accommodate particular needs as dictated by weather, crop development, and disease occurrence – always follow the manufacturer recommendations.

Although cultural controls alone won't prevent disease during seasons with wet, cool weather, the following measures will improve the chances of raising a successful crop especially with a variety that has some resistance to the disease.

1. Plant only healthy-appearing tomato transplants.
    
2. Destroy volunteer tomatoes and potatoes routinely by cultivation or herbicides. Do not let volunteers grow, even on compost piles. Infected tomato refuse should be buried or bagged and put in the trash.
    
3. Avoid wetting foliage when irrigating, especially in late afternoon and evening. If possible, irrigate in the morning or mid-day.
    
4. Space, stake, and prune tomato plants to provide good air circulation.

The Cornell web site has an informative write-up and photographs regarding late blight on tomato: http://vegetablemdonline.ppath.cornell.edu/factsheets/Potato_LateBlt.htm

Cucurbit viruses. The viruses commonly infecting cucurbits (cucumbers, melon, squash, pumpkins, watermelon) in the U.S. include beet curly top virus, cucumber mosaic virus, watermelon mosaic virus 2, squash mosaic virus, squash leaf curl, and tomato spotted wilt virus.

Although cucurbit viruses are generally not believed to be a significant problem in the Northwest several are listed in the PNW Plant Disease Control Handbook. These include, zucchini yellows mosaic virus (ZYMV), watermelon mosaic virus 2 (WMV-2), beet curly top virus (BCTV), and cucumber mosaic virus (CMV). WMV-2 has been reported from eastern Oregon and BCTV is typically limited to production areas east of the Cascades.

With the exception of SqMV, which is seedborne in melon and also transmitted by beetles, the major cucurbit viruses are transmitted by aphids in a nonpersistent manner, and also transmitted easily by mechanical means. SqMV is carried within the seed and cannot be eliminated by hot water or chemical treatment with tri-sodium phosphate. Symptoms of SqMV on young seedlings consist of pronounced chlorotic mottle, green veinbanding, and distortion of leaves. Control measures include selection of disease-free seed and cucumber beetle management. CMV and ZYMV also have been reported to be seedborne.

It is difficult to tell cucurbit viruses apart based on symptoms, and serological or molecular testing is needed to differentiate them. Infected plants may be lighter green than normal and stunted. Leaves may show intense dark green mosaic, blistering, and hardening, suggestive of a hormonal herbicide effect. Light mosaic pattern may later develop into distinct dark and light green areas on leaves, or leaves may be malformed, puckered, or blistered. Infected fruit may be mottled or have knobby overgrowths and distortions. For photos of symptoms, view the Link to Complete Cucurbit Viruses Photo Gallery at: http://vegetablemdonline.ppath.cornell.edu/PhotoPages/Cucurbit/CucViruses/CucVirPhotoList.htm

Control of cucurbit viruses often encompasses control of vectors; mineral oils, pyrethroids, reflective mulches, and nettings are used for this purpose in some areas as well as the eradication of virus reservoirs. Destruction of infected crops after harvest and strict sanitation in greenhouse crops are still recommended, but they are only partially effective and must be repeated annually.

The following adapted from Vegetable Diseases and Their Control by Sherf & Macnab, 1986, John Wiley & Sons.

Other descriptive information can be found at:

http://plant-disease.orst.edu/disease.cfm?RecordID=1036.00000 http://vegetablemdonline.ppath.cornell.edu/factsheets/Viruses_Cucurbits.htm>

Benlate Cancellation

On April 19, Du Pont announced its plans to discontinue manufacturing the fungicide benomyl and to phase out sales of the product Benlate. Benomyl is a benzimidazole compound used as a systemic foliar fungicide for the control of Botrytis, Sclerotinia, and a number of other fungal pathogens. A press release by Du Pont on this subject is at: http://www.dupont.com/corp/whats-new/releases/01/010419.html

Upcoming Vegetable Events

Check out the Vegetable Pathology Team's calender of upcoming vegetable events: http://mtvernon.wsu.edu/path_team/vegpath_team.htm