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

Manganese Toxicity in Cantaloupes

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

Growers, especially on the Eastern Shore, may already be seeing leaf symptoms on their cantaloupe plants that are often misdiagnosed as a foliar disease. However, these leaf symptoms described below indicate manganese (Mn) toxicity which is related to low soil pH.

Symptoms: Symptoms of manganese toxicity usually appear on older leaves of cantaloupe when fruit begin to net or when fruit are the size of billiard balls and there have been heavy rains. The worst symptoms appear shortly before harvest and in lower areas of the field. The best way to determine whether you have Mn toxicity is to take an affected leaf and hold it up to the sun. Tiny pin-hole sized lesions with yellow halos clustered between the veins will be visible (Fig. 1). As the lesions mature, they will coalesce, and turn brown (Fig. 2). Some cantaloupe rows often seem to be worse than adjacent rows. Affected plants frequently appear as clusters in the field. Moderately to severely affected cantaloupe plants will demonstrate poor vegetative growth and reduced or incomplete fruit maturation. The combination of all these symptoms often can be confused with several infectious diseases. Because of the symptoms growers will at times increase their fungicide sprays, which may lead to phytotoxicity problems.

Figure 1. Pin-head lesions surrounded by a halo of yellow or clear tissue.

Figure 2. Younger leaves with pin-hole lesions (red) and older leaves with pin-hole lesions coalescing to form larger necrotic areas (black).

Cause: Manganese toxicity is caused by soil pH levels that are at or below 5.8. Excess soil acidity allows manganese that is normally bound to soil particles to be released and taken up by the plant in very high concentrations, i.e., toxic levels. Manganese levels of 800-900 ppm and above in foliar tissue is usually toxic. Losses to manganese toxicity can be severe. The apparent “spread of the disease” is due to plants in the field where pH is lower developing symptoms first and plants in areas where the pH is not as low developing symptoms days or even weeks later. Growers may have had their soil tested and had spread lime in the fall but still have this problem—low pH in some parts of the field.

One of the reasons for the drop in pH even though lime has been applied is the use of pH lowering fertilizers such as ammonium and urea. These acidifying fertilizers can have a long-term effect on soil that is cumulative and leads to lower pH levels. Ammonium sulfate, (NH4)2 SO4, can significantly lower pH, while ammonium nitrate (NH4NO3) and dried blood make soil moderately more acid, and urea makes soil only slightly more acid. Ammonium is made up of nitrogen and hydrogen and over time is converted to nitrate by soil bacteria, the warmer the soil, the faster the conversion. During the conversion to nitrate, nitrogen loses hydrogen and adds oxygen. The hydrogen ions are free in the water solution between soil particles to react with various substances. Plants have difficulty obtaining the nutrients they need in the proper amounts when the soil water solution has too many hydrogen ions (low pH).

Symptoms of Mn toxicity are worse when there are heavy rains because of the lack of soil oxygen, which results in changes in the availability of some nutrients like manganese. Under saturated soil conditions manganese is made more readily available to plants and in low pH soils the likelihood of manganese toxicity increases.

Magnesium (Mg) deficiency is also a possibility when pH levels drop below 5.8. In this case plants do not take up enough of the nutrient. Deficient plants exhibit interveinal chlorosis (yellowing or scorching of leaf tissue between veins) with the veins remaining green (Fig. 3). If soils are acidic and low in Mg, dolomitic lime can be used in the fall or to help right now magnesium fertilizers can be used.

Prevention: Soil acidity levels should be maintained above a pH of 6.3. Soil tests on sandy soils need to be done every year, at least for pH levels. The pH levels can change even after one year on sandy, low organic matter soils. Lime should be mixed into the soil at least several months before planting. While many plants do not grow well in acidic soils, cantaloupe is especially sensitive to the lower pH levels. Watermelon will rarely show signs of Mn toxicity even at a low pH. There is little that can be done to correct for manganese toxicity during the season. However, using fertilizers with a nitrogen source of nitrate-nitrogen (calcium nitrate and potassium nitrate) instead of ammonium-nitrogen may help increase soil pH. Potassium carbonate also can raise soil pH. It is water soluble and can be applied through drip systems. However, correcting soil pH can be an arduous and lengthy process and it’s probably too late to see a yield response in the current season if the symptoms have already been observed.

Figure 3. Magnesium deficiency in cantaloupe with interveinal scorching and veins that remain green.

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

Managing Spider Mites in Soybeans and Vegetables

David Owens, Extension Entomologist, owensd@udel.edu and Bill Cissel, Extension Agent – Integrated Pest Management; bcissel@udel.edu

We found our first two spotted spider mites in watermelon, cantaloupe, and soybean last week. Spider mites move into fields in 3 ways: hitching a ride on equipment or workers, crawling over soil, or by being transported by wind. When host quality declines, TSSM will move to high objects and stand on their ‘tip toes’, waving their first pairs of legs until a passing air current catches them.

Spider mites quickly reproduce with the warm weather that we have had. Mites can develop from egg to adult in as few as 8 days, and lay over 140 eggs during their 2 to 3-week life span. Eggs take about 6 days to hatch. Even with the wet weather we have had, it is possible that spider mites may need treating at some point in your fields later this season. Humid weather favors entomopathogenic fungi which can reduce mite numbers, but many fungicides slow down or prevent fungal growth.

Action thresholds for watermelon are 20 – 30% infested plants with 1 – 2 mites per leaf. For R-stage soybean, thresholds are 20-30 mites per leaflet and 10% of plants with one third of the leaf area stippled. For tomato, NCSU entomologist Dr. James Walgenbach recommends an action threshold of 2-4 mites per leaflet. On tomato, spider mites can also be a contributing cause of ‘gold-fleck.’

NCSU research indicates that two spotted spider mite resistance is unstable. If mites are resistant at the end of one season, that resistance will not be as strong at the beginning of the next. However, resistance is still present and can be rapidly selected for. There is limited population mixing from one farm operation to another. What this means is that a population of mites on one farm does not spread to others. This means that miticide rotation is extremely important. Miticides tend to be most active on certain demographic groups, meaning that there is likely a subset that will not be killed from the application. Given their life span (6 days as eggs, 8 as juveniles), if you need to spray, anticipate a second application.

The table below lists the miticide active ingredients and their mode of action group for watermelon. There are generic formulations of some of the chemicals, this list is not meant to serve as an endorsement.

Miticide Active ingredient MOA group Life stage active Applications/season
Agri-Mek Abamectin 6 Mobiles (translaminar) 3-5
Gladiator Zeta-cypermethrin + avermectin 3 + 6 Mobiles (translaminar) 3
Acramite Bifenazate 25 Mobiles, some ovicidal (contact) 1
Kanemite Acequinocyl 20B Mobiles, some ovicidal (translaminar) 2
Oberon Spiromesifen 23 Eggs and juveniles (contact) 3
Portal Fenpyroximate 21A Mobiles (contact) 2
Zeal Etoxazole 10B Ovicidal, juveniles (translaminar) 1

Please note that avermectin is in the same mode of action class as abamectin. Do not apply one right after the other. Also, some of these products will stop mite feeding quickly, but the mite will take a few days to die. If you have sprayed a field and see mites a couple of days later, wait another few days and resample. Also, many of these products have a long residual activity, meaning that if it is not active immediately on eggs, it should still be around once the eggs hatch. This year, we have plans to test these products at our research station.

For soybean, three materials have been previously recommended: bifenthrin (a pyrethroid), Agri-Mek 0.7SC, and Zeal 2.88SC. Generally speaking, pyrethroid resistance can be an issue with spider mites. This class will also remove beneficial predatory insects.

As always, read the label thoroughly for further guidance. Some products have restrictions on reapplication interval, and restrictions on consecutive applications. There are also requirements on some for spray adjuvants to avoid illegal residues. Good coverage is key for miticide efficacy, even on those that are translaminar.

Watch for Aphids in Melons

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

Several reports lately of very deformed watermelon plants, but also of cucumber and cantaloupe plants. These distortions (Fig. 1) are being caused by melon aphids Aphis gossypii Glover in most cases. Melon aphids are small and range in color from a light/dark green mottle (Fig. 2), which is most common to whitish, yellow (seen during hot, dry weather), pale green, and dark green almost black forms. The legs are pale with just the tips of some parts black. The cornicles also are black. One trait of melon aphids that make them particularly difficult to manage is that unlike other aphids, their populations do not fade with higher summer temperatures. Immatures look like adults, only smaller.

Female melon aphids give live birth to clones of themselves during spring and summer and their populations can increase very rapidly especially when hidden on the underside of foliage. One of the things to look for to see if you have an actively growing aphid population is white cast skins of the aphids. Aphids must shed their skins to grow so lots of skins show that the aphids are actively growing (Fig. 2, orange arrows). The faster they grow the faster they become adults and can begin to reproduce. The reproductive period lasts about two weeks with a female producing 65-85 offspring in that time. The ideal temperature for reproduction is around 70-80°F., which are the temperatures we are experiencing now. There is evidence that there are host races, i.e., melon aphids reared on cotton can be transferred successfully to okra but not to cucurbits. This inability to transfer from one host to another has been shown for other crop combinations.

Melon aphids feed on the underside of leaves and can be a major problem on young plants when they feed near the tips of vines, sucking sap and nutrients from the plant. Their feeding causes a great deal of distortion and leaf curling, hindering the photosynthetic capacity of the plant (Fig. 1). The foliage may become chlorotic and die prematurely. They also secrete a great deal of honeydew which allows the growth of sooty mold and further reduces the photosynthetic ability of the infested plant. One of the other major problems with melon aphids (as with other aphid species) is that they are good at transmitting potyviruses such as cucumber mosaic virus, watermelon mosaic viruses, and zucchini yellow mosaic virus. It must be noted that these viruses are transmitted despite insecticide applications, which include oil sprays. This is mostly because the aphids can transmit these nonpersistent viruses within 15 seconds of reaching the plant.

Figure 1. Watermelon plant with heavy melon aphid population

Management
No thresholds have been established for melon aphid in cucurbits. Reflective mulches laid before planting can repel aphids from plants reducing or delaying virus transmission, until vine growth covers-up the plastic. In smaller fields, row covers can be used. Biological control can have a significant impact on aphid populations and is our first line a defense. Therefore, weekly sprays of insecticides should not be used in watermelon unless really needed. Because cantaloupe and cucumber are very susceptible to bacterial wilt disease, which is vectored by striped cucumber beetles several insecticide sprays may be necessary. However, resistance by melon aphids to organophosphates and pyrethroid insecticides is common. Using neonicotinoids for beetle control will help control aphids, but the neonics should not be sprayed exclusively and pyrethroids or other insecticide classes should be used intermittently for beetle control.

Figure 2. Melon aphids on underside of leaf

While many of the above suggestions are all good to prevent aphid problems what do you do once you have them? Organically there are not many good aphid control tactics to use once they show up. Applications of rosemary oil or insecticidal soaps or horticultural oils are options. These will have to be applied several times with thorough coverage of the foliage being critical for control of the pest. Rosemary oil will disrupt beneficial populations less so than soaps or oils. There are several synthetic controls that will work if thorough coverage is obtained. These chemical controls include: methomyl, dimethoate, acetamiprid, clothianidin, thiamethoxam, pymetrozine, flonicamid and combination products that include one of these. Be sure to read the label before applying any chemicals. It should be noted that a plant damaged as severely as the one in Figure 1 will not recover to produce a crop.

Vegetable Disease Update – June 8, 2018

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

Scab of Cucurbits
Cool wet weather is associated with several vegetable diseases. One of these is scab on cucurbits, caused by the fungus Cladosporium cucumerinum, which is favored by temperatures at or below 70°F and wet weather.

Watermelons are highly resistant to scab, and many cucumber cultivars also have resistance (see the Commercial Vegetable Recommendation Guide for a list of resistant cucumber varieties). However, summer and winter squash, pumpkin, gourds, muskmelon and honeydew are susceptible. Symptoms vary somewhat depending on the cucurbit crop. However, leaf lesions appear as water-soaked, pale green to grey or white and angular. Dead leaf tissue cracks and looks “ragged” and is sometimes referred to as shot-holed. Fruit may have sunken spots (summer squash or susceptible cucumber), or raised scabby lesions (butternut squash). Scab should be managed by using resistant cultivars when available, and rotating out of susceptible crops (cucurbits) for two years. Application of chlorothalonil is also effective.

Scab lesions on summer squash

Scab lesions on cantaloupe

Hop Downy Mildew
Hop downy mildew was found near Wye Mills, MD on the Eastern Shore on Friday. Our weather remains very conducive for this disease which likes cool temperatures and high humidity. Symptoms are angular lesions on the leaves that are brown and necrotic. Image shows the upper surface of leaves with angular lesions caused by downy mildew. Sporulation occurs on the lower surface of the leaf.

Cucurbit Downy Mildew
Please note that the pathogen that causes hop downy mildew is NOT the same as the one that causes cucurbit downy mildew. Cucurbit crops should be monitored for presence of downy mildew, however the closest reported outbreak of cucurbit downy mildew as of June 7, 2018 is in North Carolina.

Image show a screen shot of current outbreaks of downy mildew in the eastern U.S. from the ipmPIPE website (http://cdm.ipmpipe.org/scripts/map.php). Red denotes counties where downy mildew was found in the last 7 days, and green counties represent outbreaks reported more than 7 days ago.

New Technology for Reducing Transplant Shock

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

A new tool is available for reducing transplant shock. The chemical 1-methylcyclopropene (1-MCP) which is marketed as the product LandSpring by the AgroFresh company reduces ethylene production and stress on young plants. Ethylene in the plant hormone released when plants are injured or are under stress, as is common during transplanting. Excess ethylene can cause leaf drop and wilting and can increase transplant losses. The way 1-MCP works is that it has a similar molecular structure to ethylene but without the negative effects on the plant. It binds to ethylene receptors in the plant and thus blocks ethylene from causing damage.

LandSpring is labelled on broccoli, brussel sprouts, cabbage, cantaloupe, cauliflower, cucumber, eggplant, muskmelon, bell pepper, nonbell pepper, summer squash, tomato and watermelon. According to the company “When applied to seedlings 1-5 days before transplanting, LandSpring WP helps decrease transplant shock enabling plants to more rapidly establish and grow. Observed benefits include increased crop biomass due to better root and shoot development when plants are subjected to stress.in the weeks following transplantation”.

The label can be found at this site: https://agrofresh.octochemstore.com/wp-content/uploads/2017/04/LandSpring-_epa-approved-seedling-label.pdf

More information can be found at: http://www.landspring.info/

Improving Success with Early Planted Warm Season Vegetables

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

With the recent cold, windy weather as a reminder, it is important to understand the factors that affect success with transplanting early warm season vegetables. Remember that the average date of the last killing frost is around April 25 for most of the Delaware and cold weather events can occur well into the middle of May.

Earliest plantings of watermelons, cantaloupes, summer squash, and tomatoes will begin the last week in April. First transplanting of crops such as peppers and eggplant will begin in early May. One of the characteristics that all these crops have in common is that they are warm season vegetables that are sensitive to cold temperatures, both in the root zone and above ground. There has been a tendency to risk earlier and earlier plantings as growers try to hit the early market. Over the years, many of our early plantings of summer vegetables have suffered because of early cold damage and inadequate provisions to protect plants.

For early transplanted warm season vegetables, choose the lightest ground that warms up quickly. Plant higher sections in the field first. Avoid areas that receive any shade from woods or hedgerows. Early fields should be protected from extreme wind and should not have frost pockets. Rye windbreaks planted between each bed are desirable for early plantings because they limit heat transfer by wind. If no rye windbreaks have been planted, then consideration should be given to using row covers to protect the plants – either clear slitted or perforated low tunnels or floating row covers. Even where windbreaks have been used, row covers may be necessary for extremely early plantings.

Lay plastic mulch well ahead of time to warm soil. Black plastic mulch should have excellent soil contact because beds with poor plastic to soil contact will not heat up effectively. Firm beds and tight mulch are much more effective in warming soils. Do not lay plastic on cloddy soils. Make sure that there is good soil moisture when forming beds and laying plastic because soil water will serve as the heat reservoir during cold nights.

When producing transplants, use larger cell sizes and grow plants so that they have well developed roots in those cells for the first plantings. Large cell sizes will perform better than small cells in early plantings.

Careful attention needs to be paid to hardening off warm season vegetable transplants that will be planted early. Gradual acclimation to colder temperatures will reduce transplant shock. Do not transplant tender, leggy plants or plants coming directly out of warm greenhouse conditions for these early plantings.

Watch extended weather forecasts and plant at the beginning of a predicted warming trend. Monitor soil temperatures in plastic beds and do not plant if they are below 60°F. Soil temperature in beds should be measured at the beginning of the day when at the coolest. When soil temperature conditions are not favorable, wait to plant. Avoid planting in extended cloudy periods, especially if plants have come out of the greenhouse after an overcast period. These plants will not perform well. Extra caution should be taken to minimize root injury during transplanting. When transplanting, make sure that there is good root to soil contact and there are few air pockets around roots.

In years with cold, cloudy, windy weather after transplanting, we have had large losses of transplants in the field, especially seedless watermelons. It is critical to have warm soil conditions after transplanting to allow roots to grow out into the bed quickly. In cold, cloudy conditions, plants shut down physiologically, little root growth occurs, and the existing roots on the transplant do not function well. If there is any wind, plants lose more water than they can take up and they die due to desiccation. This is accelerated when the sun does come out – the first sunny day after an extended cold, cloudy period is when you will see the most wilting of weakened transplants.

If cold weather occurs after transplanting, warm season vegetables vary in their ability to tolerate adverse weather after being set out. Tomatoes will stop growth but will grow out without much damage once warm weather returns. Summer squash and cucumber transplants may be temporarily stunted but generally grow out of the condition. Watermelons will hold if they have been hardened off properly. Cantaloupes can be stunted if exposed to excessively harsh early conditions. Peppers and eggplants will not put on any root growth until temperatures are warm enough. If stunting occurs on any of these warm season vegetables, you may lose the early advantage you were seeking. In addition, remember that these vegetables are susceptible to frost damage and will be killed by a late freeze. Long-term weather records show that there still is a 33% chance of freezing weather in parts of Delmarva up to the 30 of April and a 10% chance of freezing weather up to May 10.

Current forecasts show a four-day warming trend starting May 1 (highs and lows forecasted in degrees F – May 1: 74-55, May 2: 81-61, May 3: 82-64, and May 4: 77-58 for Georgetown, Delaware). I would advise to target May 1 for early seedless watermelon planting.

Good windbreaks, tight mulch, and firm beds will lead to better success with early planted warm season vegetables.

Frequent Heavy Rains = Lots of Vegetable Disease Problems

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

I do not have to tell you that these frequent and heavy rains we have been having over the last 2-3 weeks have really increased the amount of foliar and, at times, soil diseases in our vegetable crops. In cucurbits foliar diseases such as Alternaria, gummy stem blight and an odd one Cercospora (Fig. 1 and 2) have been found causing moderate to severe defoliation in some fields that are heavy with fruit. The large fruit load puts a strain on the plant and when conditions are right (wet weather and warm temperatures) the fungal and bacterial diseases will flourish. Phytophthora sp also has been a problem in some cucurbit fields as this organism moves best by swimming in water and a very wet or temporarily flooded field is just what it needs to move around and infect the crop causing a ‘melt down’ of the plant.

Even when a grower has been diligent about applying their foliar fungicides and copper protectant sprays, we are still going to see plants become infected with foliar pathogens under the kinds of weather conditions we have had. One of these problem pathogens is Xanthomonas campestris pv. vesicatoria, which is the causal agent of bacterial spot in tomato (Fig. 3). However it is not that straight forward as there are at least 4 different species and four different races of this pest that can cause bacterial leaf spot. Preliminary work at North Carolina State University has shown that their bacterial leaf spot in many of their tomato fields has resistance to copper sprays. Based on what I have seen in some of our tomato fields I am sure we have similar problems. However, even if your bacterial spot is not resistant it still is going to spread and get worse in fields where it was already present after all the frequent rains that we have had. I know you have heard us in Extension say this before and repeatedly, but growers need to be sure to follow good sanitation and cultural practices in their vegetable fields, which will allow for better disease management.

Some good cultural controls include: Using pathogen-free seed and disease-free transplants –including hot water treatments that can be used to kill bacteria on and in seed. Good sanitation practices including cleaning all equipment used in diseased fields, sanitation of equipment can be done safely and effectively using a power washer and a commercial sanitizer. Keep fields free from volunteers plants, weeds, and cull piles. Avoid working in fields when bacterial diseases are present and the fields are wet. Bury or remove crop debris at the end of the season and rotate with a non-host crop for at least 2-3 years.

Figure 1. Alternaria (larger tan/brown spots) and Cercospora leaf spots (arrows) on cantaloupe leaves

Figure 2. Gummy stem blight on a watermelon leaf

Figure 3. Bacterial spot on tomato leaf

Cucurbit Downy Mildew Update – July 8, 2016

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

Downy mildew was confirmed on cucumber on the eastern shore of Maryland last week. No new reports of downy mildew have been received. However, continue to spray cucumber with downy mildew specific materials and spray other cucurbits with protectant fungicides. Most importantly, scout the fields aggressively for symptoms.