Bacterial Wilt Problems in Cucurbits this Year

Jerry Brust, IPM Vegetable Specialist, University of Maryland; jbrust@umd.edu

In a sentinel plot of cucurbits (cantaloupe, cucumber, watermelon, pumpkin, etc.) near Cambridge, MD on the Eastern Shore and in a few other cantaloupe and cucumber fields are some of the worst cucurbit bacterial wilt (Erwinia tracheiphila) infections I have seen in the past 5-7 years. Most of the infected plants are still small and were fed on by cucumber beetles 2-2.5 weeks ago. The first sign of bacterial wilt infection is when leaves near the base of the plant wilt and turn a brownish-gray/green and then dry up (Fig. 1). Then other leaves on the vine with those first dying leaves will begin to flag and wilt in the mid-afternoon (Fig. 2). In a few more days the entire vine will wilt. Sometimes that maybe the only problem but often another vine will start to wilt and then another until the plant is dead. After bacteria enter the plant it takes anywhere from 2-4 weeks for an infected plant to wilt and die.

So far this year about 18% of the cantaloupe and cucumber plants have begun to wilt. Normally I see 3-6% of plants wilt down at this plant size. I don’t think it was an unusually high striped cucumber beetle population (these beetles act as vectors for E. tracheiphila) although a few areas had very high numbers (15-20/plant). It appears that a greater percentage of beetles were carrying the bacteria than what we would normally encounter. In Figure 3 this level of feeding damage would usually lead to about 35-40% of the plants going down to bacterial wilt, this year it is 65-75% of plants like this going down to wilt.

Under this sort of pressure applying neonics to plants while they were in the tray or that were drenched at planting (which is usually sufficient) often will not be enough to hold back beetle transmission of the bacteria 7-10 days after treatment. Foliar sprays with pyrethroids would be needed. But how do you know when more beetles are going to act as vectors—you don’t. And that is the problem, next year do you over treat because of one outlier season or continue with what you have been doing? My guess is that this is a onetime blip that so many more beetles were infective than normal. If your cantaloupe or cucumber plants look good and do not have any more than the usual amount of bacterial wilt you can consider your striped cucumber beetle management to be good.

Figure 1. The base-leaves of an infected vine begin to wilt and then dry up and die

Figure 2. After 7-10 days leaves on the infected vine become flaccid

Figure 3. Heavy beetle feeding on cantaloupe plant

High Populations of Striped Cucumber Beetle Early This Year

Jerry Brust, IPM Vegetable Specialist, University of Maryland;jbrust@umd.edu

If you think striped cucumber beetles seemed to be in greater than normal numbers this year you are not alone. This has been a particularly bad year for striped cucumber beetles in squash, cucumber, watermelon and, lately, pumpkins. The beetles have been ravaging cucurbit fields in southern and central Maryland as well as the eastern shore. Some fields have been hit particularly hard with beetles causing 10-15% plant loss due just to their feeding. The cool wet spring we had slowed the emergence of the beetles from their overwintering sites. When you combine the delayed emergence of the beetles with the slow planting schedule of squash and cucumber because of the cooler weather you wind-up getting a massive movement of beetles into some fields as soon as there are any cucurbit plants available.

The beetles tend to mass on small plants where they eat, mate and defecate (Fig. 1). This type of frenzied activity where there are many beetles feeding on a few leaves or a small plant leads to increased chances of bacterial wilt problems (Fig. 2). The bacterium that causes bacterial wilt in cucurbits, Erwinia tracheiphila, is in the cucumber beetle’s feces. As the beetles defecate on the leaves where they are feeding the bacteria can be moved into open (feeding) wounds with water that is in the form of precipitation or dew. The more beetles feeding and opening wounds on susceptible crops like cucumbers and squash the greater the chance of bacterial wilt infection. In a few small cucumber fields I saw as much as 45% of the plants with bacterial wilt.

One additional problem with these pests and why control sprays have not worked as well as they should have under some conditions is that the beetles are consistently hiding at the base of the plant (in the plastic hole) where they are feeding on the stem (Fig. 3). Sprayers are usually set up to cover a lot of leaf canopy and often do not do a very good job of putting chemical down in the plant hole. This stem feeding can be severe enough to cause some wilting. It is hard enough to control cucumber beetles with a good cover spray, but when only small amounts of spray are reaching them down in the plastic hole they will not be controlled. On many of the farms that were hit hard with early beetle populations, beetle numbers seem to be much lower the last week or so.

cucbeetle1

Figure 1. Feeding frenzy of striped cucumber beetles-eating, mating, defecating

cucbeetle2a

cucbeetle2b

Figure 2. Small and large cucurbit plants infected with bacterial wilt. At first a few leaves or one vine will wilt and eventually the whole plant

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Figure 3. Feeding at the base of a young cucumber plant by striped cucumber beetles.

Vegetable Disease Update – July 2, 2010

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Cucurbit Downy Mildew
We are at minimal risk at the present but keep up to date by checking the ipm PIPE website http://cdm.ipmpipe.org regularly for updates. Downy mildew was found in New York state for the first time on cucumber in Erie and Niagara counties close to the Ontario, Canada infection site. The northern march of downy has been slowed. We have had some weather patterns coming north but the clear skies and plenty of UV radiation have probably been keeping viable spore number low. We are checking our sentinel plots weekly for downy mildew here in DE.

Bacterial Wilt
Bacterial wilt on slicing cucumbers was diagnosed this week. Symptoms on this planting were random wilting of several runners on 20% of the plants. Sticky strands of bacterial ooze can be seen when the cut ends of the wilted runners are touched together then slowly drawn apart. Striped and spotted cucumber beetles carry the bacteria on their mouthparts and inoculate them when they feed on the succulent stems early in the season. Bacterial wilt is not seed borne and does not persist in the soil more than 2-3 months. It is thought that the bacteria acquire the bacteria from infected weed or volunteer cucurbit hosts. Cucumber beetle control is the primary control method.

Strands of bacterial ooze from touching cut ends of infected runner and pulling them apart slowly

Potato and Tomato Late Blight Webinar for Home Gardeners
Rutgers, Penn State and Cornell University vegetable plant pathologists will be holding a Webinar on Potato and Tomato Late Blight for home gardeners on July 13, 2010 at 6:30 PM. You are encouraged to participate in this timely topic. The linked announcement has all the information to enroll. It will be a good review for commercial producers as well.

Pythium Blight or Cottony Leak on Snap Beans
Pythium blight or cottony leak on snap beans was diagnosed early this week. This disease likes the hot, humid conditions that we had before this recent break in the weather. When we go back to the humid weather again with scattered showers and irrigation this disease can be a threat. Look for the cottony white growth in the lower canopy and on pods close to the ground. There is a 24c registration for Ridomil Gold Copper (2 lbs/A) for prevention of Pythium blight in DE, MD and VA. Several applications may be necessary if favorable weather persists.

Cucurbit Powdery Mildew
Powdery mildew on cucurbits has been reported in New Jersey. Delaware growers should be scouting and begin applying fungicides for powdery mildew once 1 old leaf in 45 has been found with powdery mildew. See the article titled Powdery Mildew on Cucurbits in WCU 18:15 for suggested fungicides.

Vegetable Crop Diseases

Bob Mulrooney, Extension Plant Pathologist; bobmul@udel.edu

Tomato Spotted Wilt Virus
Tomato spotted wilt virus was indentified and confirmed today by ELISA on tomato plants from across the line in MD. We don’t see this very often but the symptoms are pretty obvious most of the time. It can be a problem when tomato transplants are produced with ornamental bedding plants, which harbor the virus. The virus is often then transmitted to tomatoes by Western flower thrips. The virus can be transmitted from weeds and perennial ornamentals as well by nine species of thrips. Young leaves are bronze colored and later develop numerous small dark spots. Growing points may die and stems of terminals may be streaked.

 

Tomato spotted wilt on tomato

Bacterial Wilt in Cucurbits*
Symptoms of bacterial wilt will vary depending on the cucurbit crop. In general, plants may wilt during the day in hot weather and ‘recover’ during cooler parts of the evening and morning. Margins and interveinal areas of leaves become necrotic which cause leaves to appear ‘scorched’. Look for beetle feeding scars on cotyledons and stems of young plants. Healthy green plants will turn chlorotic (yellow) with time and infected plants will eventually collapse and die, exposing fruit to sunscald injury. Cutting through stem tissue at the base of infected plants often reveals a coppery-tan color where the bacterium causes the vascular tissue to ‘plug up’. Control of bacterial wilt begins with controlling striped and spotted cucumber beetles which vector the pathogen early in the growing season as plants emerge. Late-season beetle control will remain important as fruit begins to mature. Late-season beetle feeding may cause injury to stems ruining aesthetic quality. For more information on cucumber beetle and bacterial wilt control please see the 2008 Delaware Commercial Vegetable Production Recommendations Guide.

Anthracnose of Pepper
Symptoms of anthracnose infection in pepper fruit include sunken, circular spots which develop blackish-tan to orange concentric rings as lesions develop. Lesions on stems and leaves appear as grayish-brown spots with dark margins and can easily be overlooked.

Control of anthracnose begins with using clean-free seed and/or transplants. A three-year crop rotation with non-solanaceous crops is recommended. After the harvest season, pepper fields should be disced and plowed under thoroughly to bury crop debris. Beginning at flowering and as small fruit begin to set, alternate maneb (M3) at 1.5 to 3 lb/A 75DF with one of the following FRAC code 11 fungicides:

azoxystrobin (Quadris at 6.2 to 15.4 fl oz 2.08F/A)
or
Flint (trifloxystrobin) 50WDGat 2 to 4 oz/A
or
Cabrio (pyraclostrobin) 20EG at 8 to 12 oz/A
or
Tanos (famoxodone + cymoxanil, 11 + 27) at 8 to 20 50WDG/A.

 

Anthracnose on pepper fruit

Bacterial Leaf Spot of Pepper*
Symptoms of bacterial spot on pepper leaves include small, brown water-soaked lesions that turn brown and necrotic in the centers. Spots may coalesce and form large blighted areas on leaves and premature defoliation can occur. On fruit, brown lesions can form which have a roughened, cracked wart-like appearance. High temperatures, high relative humidity and rainfall favor bacterial spot development. Loss from bacterial spot can be reduced somewhat by maintaining high levels of fertility, which will stimulate new growth. Applying a fixed copper (M1) at labeled rates plus maneb (M3) at 1.5 lbs 75DF/A or 8 to 10 oz Tanos (famoxadone + cymoxanil, 11 + 27) may help suppress spread. For more information on control of Bacterial leaf spot of pepper please see the 2008 Delaware Commercial Vegetable Production Recommendations.

*From Andy Wyenandt, Rutgers University