July 19, 2006

In this issue

§ Downy mildew spreads in southeast Michigan and moves into Indiana

§ Vegetable disease alert

§ Phytophthora in pepper production: How to minimize the losses

§ European corn borer and corn earworm update

§ Aster leafhoppers and aster yellows

§ Section 18 label issued for Carzol SP® use to control thrips in dry bulb onions

§ Hot weather may favor internal stem discoloration in celery

§ Regional reports

Downy mildew spreads in southeast Michigan and moves into Indiana

Mary Hausbeck
Plant Pathology

Reports of downy mildew continue to increase both within southeast Michigan and nearby states. In Michigan, the outbreak thus far is still limited to southeast Michigan. Currently, there are two farms in Monroe with a third farm near the Lenawee/Monroe County border confirmed late last week. Only cucumber fields have been confirmed with the disease thus far. The recommended fungicide programs are working. However, in the latest downy mildew case, the recommended fungicides for downy mildew had not been used and the field is nearly 100% infected. Remember, early applications of fungicide are critical when the spore load is high and weather is favorable (overcast and humid/wet). For instance, last week there were no symptoms on emerging cucumber seedlings in southeast Michigan on July 12^th. On July 18^th, the first true leaves were showing on 60% of the crop with the remainder of the crop still emerging. Downy mildew symptoms are now obvious on these first true leaves. (See Figure 1).

Based on our field study from last year, I recommend alternating these sprays:

Previcur Flex (1.2 pint) + Bravo (or Mancozeb)

Tanos 50DF (8 oz.) + Mancozeb (or Bravo)

A downy mildew outbreak has been confirmed in northern Kusciusko County, Indiana. The location of this particular field increases the risk to growers in western Michigan. Also, keep in mind that it is very likely that home gardens and other small production fields for roadside markets have downy mildew. In fact, I think you can bet that downy mildew is ravaging pockets of cucumbers growing in backyards. These fields are not being reported, and it is likely that the homeowner or grower does not realize what is happening. Such situations are being reported in a nearby region. As the season progresses, fields with uncontrolled downy mildew will produce large spore loads allowing downy mildew to spread. Spores are produced in highest numbers when the temperature is between 59° and 68°F. However, spores can be produced in low numbers at higher temperatures (up to 86°F).

High spore loads are occurring in southeast Michigan (see Table 1). Spore loads in other areas of the state are negligible. With the number of fields affected in Ohio and Ontario, it seems reasonable to assume that the spore loads in these regions are also high. As downy mildew builds in the region and creeps west, controls will need to be tightened heading into the August growing season. If you cut fungicide rates, use products other than those recommended (see above), or stretch spray intervals during wet weather, your risk of downy mildew will increase (a lot!). Trying to play catch up with downy mildew is a losing proposition! The risk of downy mildew in the state is probably too high now to stretch the fungicide interval beyond 7 days or to use the mancozeb fungicide alone. Cucumber growers in southeast and south central Michigan will need to be on a 5-day application interval to ensure the needed protection against this aggressive and fast-moving disease. Cucumber growers in the other regions of the state should consider a 5-day application interval during periods of overcast and wet/humid conditions. A 5-day interval will help protect a crop that is not emerging uniformly. While I am concerned about the cost of a 5-day application program, the spore load that is available in this state alone is worrisome.

At this point, it seems that cucumbers are especially at risk for downy mildew. There have not been any reports of this disease on melons and that is somewhat surprising. Based on what we know about downy mildew, the pathotypes that affect cucumbers also go to cantaloupes. Are we dealing with a new pathotype? That is a question that will not be answered this growing season. Therefore, protecting cantaloupes against downy mildew as previously recommended is a prudent course of action. It is less likely that watermelon, squash, or zucchini will become infected with downy mildew. However, growers and scouts must remain vigilant and watch all cucurbit crops carefully for any potential downy mildew symptoms.

Table 1. Spore trap daily totals (counts/m³/day) by location

Date	Michigan counties
July	Allegan	Bay	Monroe	Saginaw	St. Joseph	Van Buren
1	0	10	*52	12	7	3
2	2	8	115	8	2	3
3	0	5	8,933	32	0	2
4	2	8	4,345	12	0	15
5	-	2	2,448	3	0	5
6	-	2	1,045	8	5	30
7	0	5	*1,557	2	10	2
8	2	8	6,268	8	20	15
9	5	5	*122	3	0	7
10	3	0	*93	5	2	5
11	2	0	3,970	5	**	2
12	**	**	48,878	**	**	**
13	**	**	49,897	**	**	**

*Spores present but field dust prevents accurate count.

**Spore tapes available, counts pending.

Phytophthora in pepper production: How to minimize the losses

Mary Hausbeck
Plant Pathology

Phytophthora is a problem this year for pepper and cucurbit growers. The heavy rains experienced in some areas of the state are certain to cause a rapid increase in disease in fields that were showing early symptoms.

Fungicide trial

Research conducted at Michigan State University has identified fungicides that can be used to limit plant loss and fruit infection of peppers. At one time, the standard systemic fungicide mefenoxam (Ridomil Gold, Ultra Flourish) was very effective in protecting the plants and fruit from infection. However, the repeated use of this fungicide and the genetic adaptation capability of P. capsici resulted in resistant populations of the pathogen in many Michigan vegetable fields. In these cases, using Ridomil Gold or Ultra Flourish does not offer any control and alternative fungicides should be used (Table 1). Recent registrations of Acrobat (dimethomorph), and Tanos 50DF (famoxadone + cymoxanil) give growers alternatives to Ridomil Gold or Ultra Flourish and are helpful as rotational products for growers interested in using Ridomil Gold or Ultra Flourish.

Table 1. Products available for Phytophthora capsici control in peppers.

Product	Active ingredient(s)
Ridomil Gold/Ultra Flourish	mefenoxam
Ridomil Gold Copper	mefenoxam/copper hydroxide
ProPhyt/Phostrol	phosphorous acid equivalents
Kocide, Champ/Cuprofix Disperss	copper hydroxide/copper sulfate
Manex	maneb
Tanos	famoxadone + cymoxanil
Acrobat/Forum	dimethomorph

During the summer of 2005 a study was conducted at a grower cooperator’s farm in Oceana County with a history of P. capsici. Plots were established in raised beds that were covered with plastic mulch and drip irrigation. A single drip tape was installed under the black plastic mulch and was used to apply soil-applied products. Treatments applied at planting were made via injection into the drip tape. Irrigation occurred for approximately 45 minutes during the injection period and created a 12 in. wide treatment band surrounding the transplants. Foliar applications started 14 days after planting and were applied with a CO₂ backpack sprayer calibrated to deliver 50 GPA using three XR8003 nozzles. Foliar treatments were applied eight times on a 7-day spray schedule. Information on average plant loss and yields for the plot are presented in Table 2.

Table 2. Efficacy of fungicides for Phytophthora crown, root, and fruit rot of peppers.

Treatment and application technique	Plant loss (%)	Yield (lb/40 ft)
Untreated	42.1	19.2
Ridomil Gold 4SL 1 pt drip applied Ridomil Gold Copper 65WP 2.5 lb foliar application	5.4	53.1
ProPhyt 4.2SC 2.5 pt drip applied Ridomil Gold Copper 65WP 2.5 lb foliar	14.6	43.2
Ridomil Gold 4SL 1 pt + ProPhyt 4.2SC 2.5 pt drip applied Ridomil Gold Copper 65WP 2.5 lb foliar	10.8	44.9
Ridomil Gold 4SL 1 pt drip applied Ridomil Gold Copper 65WP 2.5 lb + ProPhyt 4.2SC 6 pt foliar	14.3	36.8
Ridomil Gold 4SL 1 pt + ProPhyt 4.2SC 2.5 pt drip applied Ridomil Gold Copper 65WP 2.5 lb + ProPhyt 4.2SC 6 pt foliar	10.7	43.5
ProPhyt 4.2SC 2.5 pt drip applied ProPhyt 4.2SC 6 pt foliar	13.1	43.8
Ridomil Gold 4SL 1 pt + ProPhyt 4.2SC 2.5 pt drip applied A12946 2.08SC 5.5 fl oz foliar	11.3	42.0
Ridomil Gold 4SL 1 pt drip applied Tanos 50DF 10 oz + Kocide 2000 54DF 1.5 lb alternate with Manex 4FL 2 qt + Kocide 2000 54DF 1.5 lb foliar	13.6	40.9

The untreated plots had higher plant and yield loss compared to the chemical treatments. Because of variability within the plot, here were no significant differences among the different chemical programs tested. All treatment programs were helpful in limiting P. capsici although no program completely prevented plant loss. The mefenoxam based-treatments seemed to be effective as these fungicides have not been used frequently at this site and the P. capsici is still sensitive to this product in this area.

European corn borer and corn earworm update

Beth Bishop
Entomology

The number of European corn borers caught in pheromone traps during July 11-18 remained relatively low in most locations throughout Lower Michigan. The exceptions were traps at two different sites, one in Mason County and the other in Macomb County, at which 15 and 13 moths per trap were captured. One trap at each of these sites has captured consistently high numbers of corn borer moths throughout the season, probably because these traps are located in extremely favorable habitat for European corn borer moths. Nonetheless, these numbers illustrate that corn borers may continue to fly and lay eggs in low numbers even when few are caught in pheromone traps. Growers with crops at vulnerable stages (sweet corn at tassel and beyond, peppers and snap beans with fruit) should consider the tolerance for worms or damage in their particular crop and treat accordingly. Most areas in southern Michigan are at or near the degree day accumulations needed for the second flight of corn borer moths (1450 to 1500 GDD50). Once the second flight begins, growers will need to treat all vulnerable crops and, possibly, tighten up spray intervals depending on how many moths are flying.

Out of seven corn earworm traps at five different locations throughout Michigan, a single corn earworm moths was captured at two different locations (Ingham County and Oceana County). Given such low numbers, growers should not have to apply additional insecticides for corn earworm. Growers should, however, be sure that any European corn borer insecticides are applied such that they cover the silk.

Aster leafhoppers and aster yellows

Beth Bishop
Entomology

Aster Leafhoppers numbers are beginning to increase in some fields, including carrot fields in Oceana and Mason counties. Infectivity rates are, in general, moderate in those areas (about 1 to 9% – see the Vegetable CAT Alert issue from July 12). Diagnostic Services has tested aster leafhoppers from a few more celery fields in southwestern Michigan. Infectivity rates in these fields are still very low (less than 1%) and the treatment threshold is 35 aster leafhoppers per 100 sweeps (celery). These results are from the following sites: N.E. Allegan County, N. W. Allegan County, Newaygo County, N. Ottawa County and S. Central Van Buren County.

Section 18 label issued for Carzol SP® use to control thrips in dry bulb onions

Walter Pett
Entomology

The Environmental Protection Agency (EPA) and the Michigan Department of Agriculture (MDA) has issued a Section 18 label for the use of Carzol SP® (Gowan®) for the control of thrips in dry bulb onions for Michigan onion growers. The label is valid from July 18 through August 31, 2006.

Growers should pay special attention to the plant back restrictions and to the application rate and maximum number of applications when using this product. See the Federal and Section 18 label for specific guidelines and directions or contact the MDA (www.michigan.gov/mda).

Hot weather may favor internal stem discoloration in celery

Mathieu Ngouajio,
Horticulture Department

Last summer (2005), Internal Stem Discoloration appeared on celery in Michigan and caused significant losses to many growers. The losses seemed to vary with the cultivars used, with ‘Dutchess’ being one of the most susceptible varieties and ‘Green Bay’ being one of the most tolerant varieties.

A similar (or nearly similar) problem has been reported in Florida in the past where it occurs about once every four to five years. It is not clear if the problem observed in Michigan is a physiological disorder or caused by a pathogen. In Florida, the problem is called Brown Stem, and a bacterium (Pseudomonas cichorii) was identified as the causal agent. Because of the sporadic nature of the disorder/disease, environmental factors may play a key role.

Last year, Internal Stem Discoloration occurred on celery plantings that were at their exponential growth phase (active growth) during the month of July. A look at the weather data indicates that during that time air temperature exceeded 90°F for a couple of days (see graph). Starting this past Saturday (July 15, 2006), air temperature has exceeded 90°F, a condition similar to 2005. Celery growers are advised to check their crop on a regular basis for symptoms of this problem. They should pay special attention to fields planted in the first half of June. If the problem occurs again this year, MSU specialists would like to take samples that could help better understand the causes. Experiments are currently being conducted in the field to determine if air temperature is implicated in the incidence of celery Internal Stem Discoloration in Michigan (see photo).

Regional reports

1 - SW Michigan Research and Extension Center

Ron Goldy

Weather

Temperatures through the period averaged slightly above normal with highs between 80°F to 91°F and lows from 65°F to 71°F. The area received from 1.5 to 4 or more inches of rain. SWMREC recorded 2.2 inches in two events on July 14 and July 18. Heavier amounts of rain were generally in the southern parts of the region.

Commodity reports

Early cucumber, zucchini and yellow squash fields are finished and are being removed or made ready for a second planting. Some wind damage to squash and pumpkins occurred during recent storms. Phytophthora symptoms are showing up in some fields, primarily at the ends of rows. Bee activity is strong in these fields.

General bell pepper harvest is scheduled to begin this week. Recent storms did knock off some flowers and young fruit.

Tomato harvest from early tunneled fields has begun. Larger volume harvest is still several days off.

Sweet corn volume and quality is increasing as mid-season varieties come into production.

Recent rains will no doubt increase disease pressure in many plantings. Growers, however, need to be cautious when spraying in hot weather. Spray applications should be made early morning or later in the evening.

2 – Grand Rapids Area

Amy Irish-Brown

Weather

Air temperatures have once again been very much above normal, and additional rainfall on July 17 has put soil moisture levels at above normal amounts.

Commodity reports

Summer squash harvest continues. Squash bug eggs and small nymphs are becoming easier to find. Controls are needed to prevent damage. Squash vine borers continue to fly and controls should be applied soon.

Staking and tying continues in tomatoes. Plants continue to look healthy with no noticeable foliar diseases. No local harvest yet. A few small hornworm larvae continue to be found.

Radish planting and harvest continues.

Lettuce crops are in various stages of growth. Harvest is ongoing.

Cabbage looks good. Cabbage moths can still be seen flying around fields.

Celery is developing nicely. Continue to monitor leafhoppers, and control them where needed to prevent the spread of aster yellows. Current infectivity rates are low. It was noted on the conference call for this Vegetable CAT Alert issue that the very hot weather recently is similar to last year’s conditions, and we could see that internal blackening again in Duchess like we saw last year. Since we don’t really know what causes it, other than blaming the high temperatures we had last year, unfortunately, this will be a good test to see if it is indeed a result of temperature extremes. See the article in this issue by Mathieu Ngouajio for more information.

Onions are beginning to bulb. Be on the lookout for thrips.

Early planted sweet corn harvest is going well. Quality is good. Corn borer trap numbers are down between generations, but larvae can still be found in some areas, and control measures are needed. Corn earworm will also be a concern. Be sure to protect the silks when they appear for earworm.

Seeded vine crops are filling rows nicely. Squash vine borers are flying, so management applications should be made soon if not already. Striped cucumber beetles can still be found in many fields, but they seem to be declining or controlled.

3 – Oceana County

Norm Myers

Weather

The straight-line winds we experienced the afternoon of July 17 caused major disruption of the power grid. High winds devastated the fruit industry in the central part of the county, especially the cherry industry.

Damage to the vegetable industry was much less severe. Irrigation was disrupted in many places, but all parts of the county received at least a half an inch of rain so that is a minor consideration.

Commodity reports

In asparagus, some fern was pushed over but doesn't appear to have broken, so damage may be minimal. Disease Severity Values had already jumped before the storm due to the heat over the weekend, and with the windblown sand, I am recommending a fungicide covering for purple spot control.

Disease Severity Values also jumped in carrots, and cercospora blight is present in many fields, so a coverage there may also be a good idea. Aster leafhopper numbers have also jumped.

Harvest in early processing and fresh zucchini fields got underway last week, and harvest in the main processing crop will begin this week. We have yet to catch an aphid in the suction crop in Mason County, but I did send two suspicious plants to the lab yesterday.

Winter squash and pumpkins generally look good with a few flowers showing up, but phytophthora is beginning to show up in some places in vine crop fields.

In sweet corn and snap beans, European corn borer traps are down to only one moth, but I did catch a corn earworm moth, my first of the year.

4 – Mason-Newaygo counties

Jim Breinling

Weather

Above normal temperatures and adequate moisture resulted in excellent growing conditions for vegetable crops in the area.

Severe storms in the area during the afternoon and evening on Monday (July 17) generally missed the vegetable growing area in the counties. There were some high winds and rainfall amounts ranged from 0.25 to 0.5 inches in Mason County and 0.5 to 1.25 inches in Newaygo County.

Commodity reports

Carrot root development is at 1-inch diameter and 12 inches long in first field for harvest, which will possibly be in the next 7 to 10 days. Weather conditions have been very favorable for leaf blight development.

Onion crop development continues with some bulbs sizing up to 1.5 inches in diameter.

Vine crop development has been rapid this past week. Slicing cucumber harvest began this week. Fruit set has begun in processing butternut squash with fruit 4 to 6 inches in length. Harvest of summer squash is now in full swing.

European corn borer trap counts persist at the Mason County snap bean site with 4, 0, and 40 on Monday (July 17).

5 – Macomb, Lapeer, St. Clair counties

Hannah Stevens

Weather

Three days of brutal heat on Friday, Saturday and Sunday (July 15-17) coupled with wind on Monday have been hard on many crops due to high ET rates and physical injury. Temperatures reached nearly 99°F on Sunday. Four days of rain brought about 1.5 inches to central Macomb with lower numbers further north at Lapeer and St. Clare county weather stations.

Commodity reports

Early bare ground sweet corn began to appear at farm markets on Monday with plasticulture corn coming to market the previous week. We have not had much of a break from corn borers this season and, with numbers jumping to an average of 12 per trap this week, we may be into the second flight already. Fortunately, corn earworms have not reached the area yet. The heat has pushed the midseason corn, and there will be plenty for customers in the next few weeks.

In squash, melons and cucumbers, striped cucumber beetles may be found easily and the heat and wind is resulting in scattered plants wilting due to bacterial wilt.

Carrot harvest is due to begin shortly. Thrips numbers on onions are climbing, but thunderstorms might have brought the numbers down somewhat.

6 - Monroe County

Paul Marks

Weather

Rain in Monroe County again this week fell heavier in the south areas than in the north. One grower in the south reported to me he has received 7 inches of rain in the last 10 days while the majority of our area has been more fortunate and received moisture as needed. Accumulated GDD50 now totals 1450 for the season, about three calendar days behind the 30-year normal for this date. Timely rains this season have apparently helped hold down any aphid populations in our area.

Commodity reports

Cucumbers are being harvested heavily now with good quality being packed. The bigger news in our area this week is the rampant spread of downy mildew, and its appearance is now common in most cucumbers being harvested, even those that have been protected on a regular basis. Younger plantings do not show the symptoms.

Pumpkins and hard squash have not shown any downy mildew symptoms and are now vining heavily. Small fruits are beginning to develop.

Tomatoes for processing have filled the twin rows with earlier varieties filling between the beds. Fruit set appears normal with limited bacterial diseases present in most fields.

Sweet corn harvest is in full swing. My corn borer traps were empty again this week; however, with 1450 degree days I would expect activity this next week.

Peppers have finally made some very good growth. Fields planted earliest have nice size fruits present, but very little thickness yet developed. Banana peppers do not yet show any of the CMV we saw last season.

Early potato harvest is progressing at a very slow rate. Early blight is common in this crop, but I have not seen any late blight in our area. Leafhopper populations have been slow to rebuild with recent rains no doubt helping with this control.