May 2001 Newsletter

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WSU Vegetable Pathology Team Newsletter

In this Issue

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. 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 

Oregon State University On-line Guide to Plant Disease Control

The Ohio State University Extension Vegetable Crops Team

IPM for Greenhouse Growing 

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.

Tomato entry	Disease progress on foliage, 7/6-9/8	% Fruit blight1 10/9
S-193	9 a	90 def
S-194	11 a	93 efgh
S-183	12 a	70 cde
NC	14 ab	15 a
NC 58LB-1 (99)	16 ab	37 ab
NC 55E-1 (99)	16 ab	42 ab
S-196	17 ab	85 def
OSU 188	12 ab	97 fgh
OSU 192	26 bc	98 fgh
S-180	28 bc	62 bc
OSU 195 (Legend)	33 cd	93 efg
Matt’s Wild Cherry2	34 cd	5 a
NC 109LB-1A	38 cd	98 fgh
Pruden’s Purple	38 cd	80 cde
NC 109LB-1 (99)-Bk	44 de	87 efg
Yellow Currant	45 def	37 ab
Grape-1	53 efg	25 ab
Sassy	56 efgh	99 gh
Santa	57 efgh	27 ab
Plum 11	58 fgh	96 fgh
Red Currant	60 ghi	30 ab
Plum 9	62 ghi	100 h
Plum 20	64 ghij	68 bcd
Early Girl3	70 hij	98 fgh
Juliet	73 ij	32 ab
Siletz.	77 j	98 fgh
LSD (P=0.05)	13.94	17.3

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.

Late blight management practices for garden tomatoes are in the PNW Plant Disease Handbook

The Cornell web site has an informative write-up and photographs regarding late blight on tomato

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.

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. 

Transmission methods

Virus	Number of Plant Families	Mechanical	Cucumber Beetle	Seed
Beet curly top	19	Yes/No	No	No
Cucumber mosaic	31+	Yes	Yes/No	Yes/No
Squash mosaic	4	Yes	Yes	Yes
Watermelon mosaic -1	1	Yes	No	No
Watermelon mosaic -2	4+	Yes	No	No

For information about testing for plant viruses using the WSU ELISA Testing Laboratory

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.

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