Striped Cucumber Beetle and Bacterial Wilt

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

Striped cucumber beetles (Acalymma vittatum) (SCB) are the most important insect pests of muskmelon and cucumbers in our area. They overwinter as adults and emerge when temperatures reach 54–62°F at which time they begin searching for cucurbit hosts. Volatiles produced by the plant attracts SCB to cucurbits initially, then male SCBs produce an aggregation pheromone attracting more beetles. The beetles tend to mass on small plants where they eat, mate and defecate (Fig. 1).

Figure 1. Early season feeding of SCB on cucumber

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 development. 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 that are feeding and opening wounds on susceptible crops like cucumbers and cantaloupe the greater the chance of bacterial wilt infection. The bacteria multiply and block plant xylem, restricting water flow to the rest of the plant; plants wilt and eventually die (Fig. 2). The wilting usually starts with just one heavily chewed upon leaf wilting and then this wilting progresses to the stem of the leaf and then to major vines of the plant. This process of vines and the entire plant wilting down can take 2-6 weeks after initial infection, but because the non-infected parts of the plant continue to grow growers might think when they see a plant wilt down that infection took place just within the last few days (Fig. 3).

Figure 2. Cantaloupe plant killed by bacterial wilt infection

One additional problem with SCB and why control sprays may not work as well as they should 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. 4). Sprayers are set up usually 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.

Melon cultivars have different susceptibilities to bacterial wilt infection. Watermelon is almost immune to infection while squash and pumpkin are moderately susceptible. Cantaloupe and cucumbers as well as some of the specialty melon types are much more susceptible. Among the most susceptible cultivars are, Honeydew 252 and HD150 which are honeydew melons; Da Vinci which is a Tuscan type melon and Miracle and Sheba which are a netted yellow-green melons. Among the most tolerant cantaloupe cultivars are Aphrodite, Athena, Accolade and Astound which are all eastern cantaloupes and just happen to all start with A. The management methods that are recommended for bacterial wilt control for standard cantaloupe varieties (using seed treatments and insecticides when beetles reach 1 per plant or using kaolin clay or row covers before beetles appear) work well. For the specialty melons more attention is needed to carefully follow management recommendations.

Figure 3. Only the leaves at the base of the plant (arrows) were initially infected with E. tracheiphila but the whole plant eventually will die.

Figure 4. Striped cucumber beetle feeding damage at base of small plants

Bacterial Fruit Blotch on Muskmelon and Watermelon

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

There have been a couple of positive confirmations of bacterial fruit blotch over the past two weeks in the region. Scout for the appearance of small water soaked lesions on watermelon or muskmelon leaves. However, be aware that plants can harbor this pathogen and not show symptoms until harvest.

Some research-based information that we have on bacterial fruit blotch (BFB) is that it will only spread from infected transplants to plants during flowering and shortly afterward. In addition, spread of BFB will occur most rapidly under warm, humid conditions and during rainfall or overhead irrigation. When the bacterium is deposited on the watermelon flower, it can penetrate through stomates and infect fruit. The infections that cause fruit loss can only take place during flowering and fruit development before wax deposition (wax seals the stomates). That means that the yield damaging infections occur only during flowering and for about 3 weeks afterward. Although infections occur early in the season, fruit symptoms often do not develop until harvest.

Chemical treatments (i.e. copper) to protect the crop should be applied before and during flowering, and for three weeks afterward. If subsequent harvests are anticipated, those fruit will also need to be protected. Actigard also can be beneficial if applied prior to flowering (see the Mid-Atlantic Commercial Vegetable Recommendations for additional information). I recommend that the workers work in uninfected fields first and any suspect fields last in the day. They should shower and wash their clothes and shoes before going into another field the next morning.

If the pathogen is introduced into the field it will survive for a short time as debris on plastic. If plants are removed and clean transplants are placed in those holes, infection could occur. The soil can remain infested for two years. However, be aware that if volunteer plants are present in the field in the intervening year, the pathogen may survive longer.

A frequent question that I get is how to clean a greenhouse after a BFB outbreak. Discard trays, wash surfaces with a greenhouse cleaner that indicates that it is a bactericide (for example: quaternary ammonium compounds such as Green-Shield®, Physan 20®, and KleenGrow™; hydrogen peroxide & peroxyacetic acid such as Sanidate®; hydrogen dioxide such as ZeroTol® 2.0, Oxidate® 2.0; or chlorine bleach) etc.

Another question is if equipment can carry the pathogen from one field to another. The biggest risk is taking debris across a field on wheels that crush the foliage. To reduce field to field spread, remove any debris stuck on the truck or equipment and wash the truck with soap and water. If you are concerned about the wheels, they can be washed with the greenhouse sanitizers described above.

Cucurbits at Risk for Downy and Powdery Mildew

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Note: Please read labels carefully as some of the fungicides mentioned in this article are not labelled on all cucurbits.

Powdery Mildew on Cucurbits
Powdery mildew on cucurbits is now beginning to progress. The powdery mildew pathogen is windborne and, unlike many other pathogens, can efficiently infect during dry periods such as we’re experiencing. Effectively managing powdery mildew requires fungicides. However, because the pathogen is prone to resistance, fungicide resistance within the pathogen population (Podosphaera xanthii) must be considered. Bioassays to evaluate the presence of resistance throughout the region have been conducted in the past year. Quintec (FRAC 13), Luna products (FRAC 7), and Vivando (FRAC U8) were all highly effective throughout the Mid-Atlantic and Northeast. Torino (FRAC U6) was effective in some locations but moderately effective in others including in my Maryland trials. Remember that the FRAC group 11 and 1 fungicides, which include strobilurin fungicides like Flint, and Topsin M, are ineffective. FRAC group 3 and 7 fungicides, which include Myclobutanil (Nova), boscalid (one of the active ingredients in Pristine), Fontelis and Folicur are in an intermediate group. We do know that resistance to FRAC groups 3 and 7 can be found in our pathogen populations, but if these products are used judiciously and in rotation with other effective products, they can be useful. Always tank mix fungicides with broad spectrum materials such as chlorothalonil, and alternate with a fungicide that has a different mode-of-action (FRAC group). A good strategy is to use moderately resistant cultivars and then alternate fungicides in FRAC groups where resistance has not been detected with fungicides in FRAC groups 3 or 7.

Pumpkin leaf that has both white powdery lesions, and brown downy mildew lesions. It is important to be sure that you are spraying for the correct disease because the most effective fungicides for each disease are different.

Downy Mildew on Cucumber
Downy mildew on cucumber has now been confirmed in Salem County, NJ. All cucumbers should be protected with targeted fungicides. Other cucurbits should be scouted aggressively for the presence of downy mildew.

Lack of Netting in Muskmelons

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

Recently a muskmelon field was observed with a whole section of the field producing fruits without netting (bald).

Melons have several different potential rind patterns: smooth such as honeydews, wrinkled such as canary melons, or netted such as our eastern muskmelons and western cantaloupes. Netting is controlled genetically and is highly heritable. Breeders select for netting types in their programs when developing new melons.

Commonly we find lack of netting in muskmelons where fruits have not fully developed due to poor pollination and late in the season when nights are cool affecting fruit development. There is also an association with calcium levels and netting. Poor netting can be a result of calcium deficiencies under low pH soil conditions.

Vegetable Crop Disease Update – August 5, 2016

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Downy mildew is active in both Maryland and Delaware on cucumber and cantaloupe (muskmelon). Muskmelon was first reported in the past week.

Powdery mildew is also present on cucurbit crops. Stay on a good fungicide management program.

Late blight is also present in four counties in Maryland, including on the shore adjacent to Delaware. Recent hot weather has slowed the spread of late blight, however as we move into a period of cooler temperatures and longer nighttime dew periods, late blight is likely to increase in prevalence again.

Gummy stem blight on watermelon (Fig. 1A) and melon (Fig. 1B) and anthracnose on watermelon (Fig. 2, note leaf also has gummy stem blight lesions) are also widespread.

GummyStemBlight

Figure 1. Gummy stem blight on watermelon (A) and muskmelon (B)

WManthracnose

Figure 2. Anthracnose (and gummy stem blight) on watermelon.

Fungicide Products for Managing Powdery Mildew on Cucurbit Crops

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

There are many new fungicide products for management of powdery mildew on cucurbits such as pumpkin, squash, cucumber and melon. Last year we conducted a trial at the University of Maryland’s research farm in Salisbury that included some of these new fungicides (and a few “old” fungicides). The fungicides tested in this trial are in Table 1. Note that some of these fungicides, such as Quintec, are not labelled on all cucurbits, so check the labels carefully.

Table 1. List of fungicides and Fungicide Resistance Action Committee (FRAC) Code tested in 2015.

FRAC Code M5 (Contact fungicide) Bravo Weatherstik
FRAC Code 3 Proline
FRAC Code 3 Procure
FRAC Code 7 Fontelis
FRAC Code 3 + 7 Aprovia Top
FRAC Code 3 + 7 Luna Experience
FRAC Code 13 Quintec
FRAC Code U6 Torino

 

Several lessons could be gleaned from this trial that confirm results from other trials.

1) Always rotate fungicides with products in different FRAC Code groups. When rotated with Procure, Luna experience performed significantly better than when used without a rotation partner.

2) Products that have some systemic activity perform better than products with only contact activity. In our trial, when Aprovia Top was alternated with Bravo Weatherstik, it was only moderately effective in reduction powdery mildew. However in alternation with Procure, Aprovia Top was much better at reducing powdery mildew.

3) Using the best fungicides earlier in the season will improve control. We designed some treatments to see if applying the most effective rotation partner early or late was better for control. The differences were small, but when the better product was applied early, the control was marginally better.

The two best treatments in this trial were Luna Experience alternated with Procure, and Quintec alternated with Procure. Aprovia Top alternated with Procure, and Torino alternated with Procure significantly reduced powdery mildew, but were not quite as effective. Better than the control, but even less effective was Fontelis alternated with Torino.

Cucurbit Downy Mildew Update

Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland; keverts@umd.edu

Downy mildew on cucumber has been reported in Delaware and Maryland as of July 3. However, our region remains at high risk for onset of downy mildew on additional cucurbits species. In addition, expect further spread of downy mildew to cucumber more fields in the region.

Ozone Injury to Vegetable Crops

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

With the hot, humid weather, we are starting to see evidence of air pollution damage in sensitive vegetable plants.

Vegetables most susceptible to air pollution include potatoes, watermelons, cantaloupes, snap beans, pumpkins, and squash.

Damage is most common during hot, humid, hazy weather with little wind. Air inversions, when warm air at the surface is trapped by even hotter air in the atmosphere above, lead to build up of air pollutants that cannot disperse and, consequently, plant injury. The most common form of air pollution injury to plants is ozone damage. Ozone is a strong oxidant and is formed by the action of sunlight on products of fuel combustion. It is moved from areas of high concentration (cities, heavy traffic areas) to nearby fields.

Ozone injury in susceptible vegetable varieties develops when ozone levels are over 80 ppb for four or five consecutive hours, or 70 ppb for a day or two when vegetable foliage at a susceptible stage of growth. Because it occurs in areas with high levels of automobile exhausts, crop injury is often visible on fields in close proximity to roads, especially with heavy summer weekend traffic. High pollution indexes in Baltimore and Washington are also a good indication that ozone damage may occur.

In potatoes, symptoms of ozone damage occur on the most recently emerged leaves and can be seen as a black flecking. Early red varieties are most susceptible.

Injury on watermelon leaves consists of premature chlorosis (yellowing) on older leaves. Leaves subsequently develop brown or black spots with white patches. Watermelons are generally more susceptible than other cucurbits to ozone damage. Damage is more prevalent when fruits are maturing or when plants are under stress. Injury is seen on crown leaves first and then progresses outward. Seedless watermelon varieties tend to be more resistant to air pollution injury than seeded varieties, so injury often shows up on the pollenizer plants first. “ice box” types are the most susceptible.

In muskmelons and other melons, the upper surface of leaves goes directly from yellow to a bleached white appearance.

Ozone injury on squash is intermediate between watermelon and cantaloupe starting with yellowing of older interior or crown leaves. These leaves subsequently turn a bleached white color with veins often remaining green.

In snap and lima beans, ozone causes small bleached spots, giving a bronze appearance on upper leaf surfaces and pods. Leaves may ultimately turn chlorotic and senesce (drop).

Ozone injury can be easily misdiagnosed as mite injury, pesticide phytotoxicity, or deficiencies.

The key to avoiding air pollution injury is to plant varieties that are of low susceptibility and to limit plant stresses. Certain fungicides such as thiophanate methyl (Topsin and others) offer some protection against ozone damage.

 ozonewatermelonOzone injury on watermelon

ozonesquashOzone injury in summer squash

Chlorosis and Bleaching in Melon Leaves

Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu

We are seeing problems with melon leaf yellowing again this year. This is a common problem in muskmelons and specialty melons. There are several potential causes.

If the yellowing is on leaf edges it most commonly is due to salt effects and fungicides. Copper fungicides are often the culprit in this leaf yellowing, causing a phytotoxic reaction. Foliar fertilizer applications can often worsen the yellowing by increasing salt levels on the leaves.

Each year there are some fields of cantaloupes that are affected by manganese toxicities. This occurs when bed pH drops below 5.4 which affects soil chemistry so that plant available manganese increases greatly and plants take up quantities that become toxic. As a micronutrient, manganese is needed in only small amounts and the sufficiency range is between 20-100 ppm. Symptoms of manganese toxicity are pin hole yellow spots clustered between the veins of older leaves

melonbleaching1Marginal yellowing on cantaloupes due to excess salts (Dan Egel, Extension Vegetable Pathologist, SWPAC, Purdue University)

lope2Leaf bleaching due to reaction to copper fungicides

Magnesium deficiencies also can occur at low pH and older leaves will show interveinal chlorosis. These symptoms can be confused with mite damage so check for mites in the diagnostic process.

Air pollution is another cause of yellowing of cantaloupe leaves. This yellowing is usually confined to older crown leaves.

melonbleaching3Air pollution damage to melons

Cucurbit Downy Mildew Update – August 19, 2011

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

There were reports of downy mildew on pumpkin in northern New Jersey at the beginning of the week. This was sent to the WCU mailing list to make you aware that downy mildew was beginning to appear on more than just pickling cucumber in the area. With the recent thunderstorms, cooler nights and morning fog, conditions will be more favorable for disease development. Maintain your fungicide program at this time. Growers should be aware that the fungicides that have been the most effective on downy mildew on cucumber (namely Presidio, Ranman, and Previcur Flex) will also be very effective on pumpkin, cantaloupe and any other cucurbit. Tanos and Curzate could be added to that list as well for cucurbits other than cucumber. Be aware that Presidio has some plant back restrictions for crops not on the label. The link will take you the new supplemental label: http://cdn.extension.udel.edu/weeklycropupdate/files/2011/08/PresidioSupplementalLabel.pdf. Wheat can be planted 30 days after treatment. This was added in the supplemental label.