Vol. 20, No. 17, August 17, 2005

In this Issue
Downy mildew spreads to several counties in Michigan
Cucumber mosaic virus detected in cucurbits and peppers
European corn borer and corn earworm
Regional report
Weather news

Mary Hausbeck,
Plant Pathology

Over the last week, downy mildew has been confirmed in several regions including southwest Michigan, the Grand Rapids area, Lake Odessa area, Ingham County, Saginaw County and Gratiot County. Hundreds of acres are impacted. The majority of the acres affected are those planted to cucumbers. The disease has also been confirmed on yellow squash. At this time, downy mildew has not been confirmed on Oceana, Mason or Monroe counties.

Downy mildew is different from powdery mildew. Powdery mildew occurs each year in Michigan on cucurbits whereas downy mildew is a new problem this year. The tell-tale symptom of downy mildew is the purplish/gray fuzz on the underside of the leaf giving a somewhat “dirty” appearance. When powdery mildew occurs on the underside of the leaf it is white – not purplish/gray.

Downy mildew is well-known for causing catastrophic losses in a brief period of time. When the conditions are favorable, unprotected foliage can become completely infected and appear to be frosted within 10 days of initial infection. The worst case scenario is that once the foliage dies, the fruit stop developing, quality decreases, sun scald occurs, and secondary rots develop. In a best case scenario, if the fruit are mature enough when the foliage becomes infected with downy mildew, they can be harvested with minimal yield loss even though the foliage has completely collapsed.

Chemical control must be focused on using the most effective products, alternating the products and applying the fungicides at short intervals. The most effective spray programs include the following:

Tanos 50DF + mancozeb 5 day PHI (or Tanos + Bravo 3 day PHI)
Aternate with
Previcur Flex + Bravo 2 day PHI
Alternate with
Gavel (contains mancozeb already) 5 day PHI

The addition of mancozeb in these programs increases the PHI to 5 days. Tanos 50DF alone has a 3 day PHI and Previcur Flex has a 2 day PHI. Please note: Neither mancozeb or Gavel are labeled for use on pumpkins. The Curzate fungicide can also be used and is one of the active ingredients in Tanos. Growers who want to protect their younger plantings need to use the program outlined above and spray every five to seven days.

At this time, I cannot recommend the strobilurin fungicides or the Ridomil-based products for downy mildew control. The strobiluring fungicides include Quadris, Amistar, Cabrio, Flint and Pristine. The disease has affected too many acres in the state and the threat of resistance is too high. The downy mildew fungus can mutate and become resistant to these fungicides. For the most part, these products have not been successful enough in southern trials to warrant their use in Michigan. Acrobat has not been a strong performer against downy mildew and probably should not be used given the severe disease pressure in the state. Similarly, copper has not provided adequate protection against downy mildew.

The widespread cucumber mosaic virus has made matter worse. Many growers are trying to determine whether they should invest in downy mildew sprays given the virus showing up in the foliage and the fruit. For growers trying to coax another week or two from their virus-infected fields, applications of Bravo or mancozeb may be adequate.

As soon as harvest is complete, the remaining vines should be killed with herbicide or plowed under immediately so that they do not become infected and serve as a source of downy mildew for nearby crops. This pathogen should not overwinter in Michigan. Downy mildew has not been confirmed in Michigan prior to this year. Previously, I’ve handled samples that growers and consultants thought might be downy mildew but was not.

The downy mildew is primarily wind-borne. This pathogen overwinters in southern states that do not experience hard frost. The early hurricane season is likely responsible for the movement of downy mildew into the Michigan growing regions. The 2005 hurricane season has been noted to be unusually active compared with other years.

My lab and MSU Diagnostic Services are working together to confirm downy mildew. The diagnosis takes just a few minutes. It is important to track this outbreak so that appropriate controls can be taken by growers in the various production regions in the state. I can be reached via cell phone at: 517-927-4532.

Mary Hausbeck
Plant Pathology

Cucumber mosaic virus (CMV) has added to the misery of many growers this summer. CMV is the primary virus in many pepper, cucumber, zucchini, squash and pumpkin plantings. It has been a couple of years since the level of virus has been so high and widespread. This is a devastating disease especially when couples with drought and heat stress.

In the vine crops, yields are reduced and many of the fruit are deformed and off color. In peppers, the foliage may not show symptoms, but the yields may be reduced and the quality of the fruit is compromised. In some cases, there is blotchiness or brown flecking on the fruit making it unacceptable even for processing.

CMV is spread via aphids, which have been in high numbers this year. There are no sprays available to protect against viral infection. Reducing weeds that border the fields may be helpful since commonly occurring weeds are known to harbor the virus and allow it to overwinter. Ultimately, genetic resistance in the crop is the best protection but may not be available in the preferred varieties.

Beth Bishop, Entomology

Numbers of European corn borers moths caught in pheromone traps increased over the past week in most locations in Michigan. The second flight of moths is laying eggs on sweet corn, peppers and snap beans. Sweet corn at tassel and beyond, and peppers or snap beans with fruit should be treated with insecticide to prevent corn borer infestation

A fairly high number of corn earworms were caught in pheromone traps in Western and Southern Michigan (Figures 1 and 2). Low numbers were captured elsewhere. Corn earworm moths migrate into Michigan from more southern regions. They come into the area at all stages of maturity, and some are ready to lay eggs immediately. Corn earworm moths lay eggs on sweet corn silk. Insecticide applications made to control European corn borer will also control corn earworm as long as the silks are thoroughly covered.

Once the silk dries down and turns brown, it is no longer attractive to corn earworm. Corn earworm may also lay eggs on other crops, such as snap beans and tomatoes. These are at risk for corn earworm infestation, especially when sweet corn fields are maturing and are not attractive for egg laying. Consult Bulletin E-312 for insecticides registered for corn earworm control in these crops.

Ron Goldy

Virus symptoms are present in peppers. It is most likely cucumber mosaic virus since the typical “oak leaf” pattern is developing in the leaves.

Tomato quality has improved over the past week as growers pick through fruit that was set in late June when it was hot. Mites continue to be a problem. Thrips damage can be observed on the fruit and thrips can be found on the leaves. Symptoms are seen a small, gold, pin prick-sized spots that will be greater in number where two fruit touch or where the fruit is under a leaf.

Sweet corn volume and quality is high. European corn borer damage appears light and rust can be found in some fields.

3 – Oceana County

Norm Myers

Disease pressure has increased even more in carrots because of irrigation. Aster yellows is much less this year than the last couple of years.

Sets of winter squash and pumpkins are light because of the heat and there will be some virus problems. Growers are concentrating on keeping downy mildew off of pumpkins and fresh market winter squash.

Summer Squashes: Virus has shut down fresh production on all but the virus resistant varieties. Processors are still taking green from non-virus resistant varieties and some yellows from young fields. The rumor is that most processors are going to come up well short of their production quotas. No sign of downy mildew yet and to date there is little powdery mildew.

Snap Beans: Growers have begun to add sprays because of the big jump in Corn Earworm trap catches. There isn’t much late sweet corn left so snap beans may be in danger from this pest.

Sweet Corn: Growers of this commodity are also tightening up spray intervals because of the Corn Earworm numbers.

Jim Breinling

Fungicides have been applied to most vine crops acreage in the area for protection against downy mildew. Two samples were sent to MSU yesterday (August 16) for diagnosis of leaf spot symptoms. Sample of mini-pumpkin sent on August 11 tested positive for CMV.

Hannah Stevens

Virus symptoms continue to appear in winter squash, pumpkins and summer squash. (See pictures) While many fields are suffering there are also some that have very limited symptoms. Some of the best fields I have seen in this area are on ground that has never grown cucurbits. While this should not influence viral infection, which is spread by aphids, it limits other pest problems. Pumpkins are maturing at a rapid pace and many fully colored fruit are now visible. Powdery mildew is also easy to find. The heat pushed summer squash development and the crop is in limited supply. Growers are hoping for good development in the late crop, but concerned about viral problems.

Southern Macomb County gardeners experience sporadic outbreaks of pepper maggot. (See pictures) While this pest has never caused serious problems for commercial producers, the fruit rot is an aggravation and the pest is difficult for gardeners to manage. We are also seeing some type of maggot on early tomatoes in a garden.

Crinkled leaves have been reported in some late fields of snap beans. This is reminiscent of virus problems experienced two years ago.

Paul Marks

Cole crops are all being harvested with some very early fall cauliflower already available. Cabbage continues to be in good supply with excellent quality, especially considering the heat.

Melon harvest continues with great quality melons available. Yields have been tremendous. Warm nights and sunshine has kept quality high.

Pepper harvest is well underway with very good quality bells. Banana and jalapeno peppers in our area are being affected with cucumber mosaic virus. (See photos). Aphids have been difficult for some growers to control this season no doubt leading to this problem.

Processing tomato harvest has picked up steam this week with most growers now on a busy schedule. Crop looks good but it is too early to predict any yields.

Market tomatoes are in good supply with very good quality continuing.

Pumpkins are sizing fruits now with some color beginning to appear. Virus and powdery mildew continues to become more of a problem.

Jeff Andresen
Agricultural Meteorology
Geography

Fair and dry weather is expected statewide Wednesday (August 17) in response to a high pressure center over Ontario. The high pressure will drift east on Thursday, followed by a warm front, which will move from southwest to northeast across the state. The warm front will set off areas of showers and thunderstorms, beginning overnight Wednesday in western sections of the Upper Peninsula and ending by early Friday morning across eastern sections of the state. Rainfall totals are generally expected to be variable from 0.1- to 0.5-inch with some scattered 0.5- to 1.0-inch totals possible in northern areas of the state. A temporary lull in the chance for rainfall is likely Friday afternoon before a cool front approaches the state from the west. Shower and thunderstorms will break out ahead of the front late Friday north into Saturday afternoon and evening across the south. Given strong upper air dynamics with this system, some severe thunderstorms are a possibility. Much cooler air will move into the region in the wake of the front with some scattered showers possible on Sunday.

Temperatures will warm from highs from the mid-70s north to low 80s south on Wednesday to the low to mid-80s Friday. Low temperatures should range from the low 50s north to near 60s south through Friday. Following the frontal passage Saturday, look for highs Sunday to fall back to the 60s north to 70s south, and low temperatures from the 40s north to 50s south. Fair and dry weather is likely early next week with a slow but gradual warming trend.

Medium-range forecast guidance is currently having difficulty resolving jet stream trends beyond the 10-day time frame with a broad troughing feature across the Great Lakes region expected prior to that. Both the 6-10 day and 8-14 day outlooks, covering August 22-26 and 24-30 call for below normal precipitation totals statewide. Temperatures in the 6-10 day time frame are forecast to range from below normal in southern sections of the state to near normal levels in the north. Similarly, the 8-14 day outlook calls for mean temperatures to range below normal levels across the southern Lower Peninsula to above normal in Upper Michigan. Given dissimilarities among several of the forecast tools, forecaster confidence in the 8-14 day outlook is considered to be much lower than normal.