Root Knot Nematode on Watermelon and Other Cucurbits

Kate Everts, Vegetable Pathologist, University of Maryland;

All cucurbits, including watermelon and cantaloupe, are hosts of root knot nematodes (RKN). RKN are plant pathogenic roundworms, which live in the soil. Although many other species of nematodes are nonpathogenic to plants or even beneficial, RKN can invade host roots and result in yield losses to cucurbits. Symptoms of RKN are often overlooked because the stunting, reduced vigor and wilting of the host plant can be caused by many other biotic or abiotic causes. To determine if plants are infected with RKN, observe plant roots for large galls, knots, or swellings. Also, look for damage that occurs in patches in the field. Plants are most susceptible to damage from RKN at the seedling stage.

Preventative management of plant-parasitic nematodes, using rotation, cover crops, transplants that are free of nematodes, nematicides and soil fumigants, is an important measure and more effective than trying to manage an outbreak. Use of green manure and soil amendments is also beneficial. We have found the soil incorporation of large amounts of organic matter, such as sorghum-sudangrass green manure in combination with poultry compost, reduces populations of root knot nematodes. Some rapeseed cultivars, such as ‘Dwarf Essex’ and ‘Humus’ also are suppressive to nematode populations.

As noted above, management of RKN is best done prior to planting. Where damage has been observed in the past, several soil samples should be taken from soil within the root zone, mixed together, and sent to a diagnostic lab for identification. If RKN is present in damaging levels, be sure that appropriate cultural practices are in place. If chemical management is necessary, it is best conducted preplant with fumigants such as Telone or Vapam.

Once the crop has been established the available options are less effective. However, the following can be used, Vydate L can be applied at 0.5 to 1.0 gal/A and incorporated into top 2-4 inches of soil, or at 2.0 to 4.0 pt/A apply 2 weeks after planting and repeat 2-3 weeks later. Velum Prime, which is in a different chemical class, can be applied at 6.5 to 6.84 fl oz/A through drip irrigation at 5-day intervals (see label for details).

I discussed this problem with Dr. David Langston at Virginia Tech, who has worked extensively on nematodes. His opinion is that although both Vydate and Vellum Prime may have some effect on nematodes when applied after transplant, the effect will be modest. If a grower is committed to a rescue treatment, keep in mind that watermelons are relatively tolerant to RKN and either forego treatment, or apply the least expensive option.

No matter what the treatment decision, remember that the damage from RKN can be mitigated, to some extent, by providing plants with adequate nutrition, moisture, and protection from stress.

Pollinator Strength

David Owens, Extension Entomologist; and Gordon Johnson, Extension Vegetable & Fruit Specialist;

Honeybees are used extensively to ensure adequate pollination for vine crop vegetables (cucurbits) and for many fruit crops (apples, berries, etc.). Without good pollination, poor fruit set or misshapen fruit can occur. Most of the honeybees used for pollination are rented from beekeepers. Questions have come up how to know if a colony is strong enough to provide adequate pollination service. A good resource on pollinators, colony strength, and farmer best management practices for pollinator health (including water sources can be found in the MidAtlantic Vegetable Production Recommendations, Section A, pages 21 – 27.

It is important to ensure having enough bees (managed and wild) to avoid having problems with fruit set and misshapen fruit. There are two ways to check the strength of a colony: in-hive inspection and assessing hive traffic at the entrance. In the hive, bees should cover 6 to 8 frames, have 4 to 6 frames of brood and (eggs, larvae, and capped) fill 1.5 to 2 boxes. This is considered a ‘minimum standard.’

An easier, but less accurate method of assessing colony strength is to watch colony entrances in late morning to early afternoon on a calm day. During a 1 minute interval, 50 – 100 bees should be arriving and leaving the colony. While counting bees, be sure to note the presence of bees carrying pollen. They will have large yellow ‘sacs’ on both back legs.

Farmers should work with their beekeeper to ensure that only strong colonies are placed in fields. This has become more difficult in recent years due to higher winter mortality caused by bee pests and pathogens. Stronger colonies provide much more pollination service than one or two weaker colonies. Beekeepers should work with the state apiarist, Meghan McConnell to assess colonies. On the farm side, farmers should read labels carefully and avoid making applications when bees are active in fields. Several insecticides and miticides have pollinator advisory language on them. The fastest way to find it is to download the label from a website such as and search the label for ‘bee’ or ‘pollinator’ using Ctrl + F. Insecticides of special concern have a bee in a red diamond to indicate pollinator protection language. Bees can also be affected by fungicide applications. Bees feed their larvae fermenting pollen, and bees rely on the microbes living with them to fend off diseases; fungicides can disrupt the beneficial microbes in the colony. Thus, even fungicides should be timed for periods when bees are not active in the crop. On warm days, bees also forage for water to cool the colony. Having a clean water source within a ¼ mile will benefit the bees. This doesn’t necessarily mean flowing water; large puddles should suffice.

Pollination in Seedless Watermelons and Honey Bee Placement, Bumble Bees as Pollinators

Gordon Johnson, Extension Vegetable & Fruit Specialist;

A female watermelon flower will need around 500-1000 pollen grains to be fertilized effectively. This will require a minimum of 8 visits by a honey bee for seeded watermelons. In seedless watermelon more visits will be required. The pollen produced by seedless watermelons is not viable. To fertilize seedless watermelon, pollen must be transferred from viable male flowers in standard or special pollinizer seeded types to triploid seedless female flowers. Because bees foraging in seedless watermelon plantings carry a mix of viable and non-viable pollen, more pollination visits (16 to 24) by honey bees are needed to set fruit.

First planted watermelons are now flowering in Delaware and Maryland. Honey bees should be placed when the first female flowers appear to achieve good crown sets without defects (i.e. prominent lobes or hollow heart). Placement should be made before 10% of plants are in bloom.

The crown set in watermelon is fruit that set on one of the first 8 nodes of the plant. This is often the most profitable, especially early in the season. Poor crown sets in watermelon can occur when there is poor weather during early flowering. Honey bee flights are reduced significantly in rain and when winds are 15 mph or greater. Cloudy weather also reduces bee activity. Honey Bees also do not fly much below 55°F, so on cold mornings, as we often have in June, bee activity will not pick up until later in the morning. Unfortunately, female watermelon flowers open early in the morning, are most receptive before 10 am, and then close in the afternoon.

In addition, in early mornings and during poor weather, bees usually visit plants closest to the hives. As the temperature rises or the weather improves, the bees will forage further from the hive. This means that in bad weather watermelons closest to the hives will have the best set and furthest from the hives will have reduced set.

Another problem that causes crown set reduction is the loss of pollenizer plants due to unfavorable weather conditions during or after planting. This means that pollen will be limiting. Research has shown that were pollen is limiting, fruit numbers will be reduced with distance from a pollen source. In fields with limited pollen, expect reduced fruit set or reduced fruit size in areas where pollenizers are missing.

Watermelon growers can manage crops for improved pollination and fruit set with honey bees by:

  • Increasing the number of honey bee hives for early watermelon crops. A minimum of one strong hive per acre is recommended in general and 2 hives per acre can be justified for early planted fields.
  • Placing hives in several locations in a field rather than just on one edge. While bees will fly over a mile, the best pollination activity is closest to the hives. Hives placed within the field will provide more bee visits to the crop compared to edge placements. Place hives in groups of 4-8 in good locations throughout the field to have even distribution of bees.
  • Having ample sources of pollen by planting pollenizers at a minimum ratio of one pollenizer per every 3 seedless plants. Use the most effective pollenizers as shown by local trials. In-row pollenizers should have limited competitiveness with the seedless melons.

Bumble Bees
Compared to a honey bee, bumble bees are about 10 times more efficient as a pollinator due to their size, the speed at which they transfer pollen, the efficiency with which they gather pollen within various crops, and their increased endurance to fly in adverse weather for longer periods of time. The bumble bee also has the ability to buzz pollinate the flower for pollen, a pollination technique not seen in honey bees. Buzz pollination occurs by bumble bees vibrating the flower by pumping their wings at a certain frequency, to dislodge pollen. Bumble bee foraging activity starts earlier and ends later in the day than managed honey bees and they forage in lower temperatures. Because of these characteristics, fewer bees are needed to achieve the same crop pollination and commercial colonies only have about 200 bees each (800 per quad).

When assessing bumble bee activity, flag out 10 areas in your field and observe each area on three different days during bloom. These observations should last one minute under sunny, windless conditions, between 9 a.m. and noon. Approach each plot with care so as not to disturb the foraging bees. Stand about three feet from the crop to avoid blocking the flight path of the bees. Count and record the number of bumble bees at each flag, then calculate the average for your observations. You should an average one bumble bee per ten flags (0.1 bees per flag) to have adequate pollination.

Bumble bee colonies should be shaded and can be placed along shaded field edges. However, if there are other wild flowers nearby, they will also work in those areas, reducing their field effectiveness. Therefore, when placing bumble bees in watermelons or other flowering vegetable or fruit fields needing pollination, it is recommended that bumble bee quads be placed in the field middles under a shade canopy to have more foraging in the target field. Bumble bees should be placed far from honey bee hives to avoid honey bee pollen theft from bumble bee nests.






2019 Seedling Diseases of Watermelon and Melon on Transplants

Kate Everts, Vegetable Pathologist, University of Maryland;

There has been a report of a bacterial seedling disease on muskmelon ready for transplant this week. The sample in question turned out to be angular leaf spot (ALS) caused by Pseudomonas syringae. Recent research from University of Florida demonstrated that populations of P. syringae that cause ALS are very diverse. It isn’t uncommon to see this disease every few years in Maryland and Delaware. Our experience has been that if infected plants are transplanted to the field and the weather turns hot and dry, damage due to ALS may be minimal. However, prolonged cool and wet conditions will result in losses.

If conditions favor disease development, apply the labeled rates of fixed copper plus mancozeb. (Some coppers are OMRI-approved and may be able to be used in organic systems to help suppress ALS.) Continue applications at weekly intervals. In addition, to reduce ALS spread, avoid overhead irrigation when symptoms are present and avoid working in field while foliage is wet.

Nathan Kleczewski and I wrote an article a few years ago about several greenhouse diseases that occur during transplant production. The link to that information and many images of these seedling diseases can be found at: =6727

Vegetable Crop Insect Scouting

David Owens, Extension Entomologist;

I found a couple of aphids on my greenhouse transplants and didn’t think too much of them. I set my transplant trays outside to harden off last week and came back to them this week only to find aphids had infested several trays, causing leaf cupping. In most cases, significant parasitoid pressure was present and most aphids were already transformed into parasitoid mummies. Pyrethroids will not affect green peach aphid or melon aphids, there are numerous other products that will do the job, including neonicotinoid insecticides that also pick up cucumber beetle. Cucumber beetles were observed feeding in a field that was transplanted last week. Transplants were treated prior to setting, and beetles were affected and dying. Residual activity generally lasts between 2 and 4 weeks. If chemigating insecticide through the drip tape, figure your rate based on field footprint, not plastic footprint. For example, if a product goes out at 10 ounces/acre and a field is 10 acres, then 100 ounces of product need to be delivered. If you base the rate on the amount of actual plastic (roughly 20-30% of the field area), you could be significantly undertreating! As plants are coming out of the greenhouse, be sure to also check for the presence of two spotted spider mites. On transplants, leaf stippling will be more evident than on older plants.

Sweet Corn
The 2019 insect trapping network has largely been deployed. Trap capture data will be uploaded to the webpage as in previous years, and most recent trapping data presented here when sweet corn is closer to tasseling. You can find trap catch data here: We are picking up low numbers of corn earworm, most likely from overwintering pupae and at this point are more of a curiosity. Scout for black cutworm damaging seedlings (3% cut plants or 10% leaf feeding).

Vegetable Disease Results from 2018 Field Trials

Kate Everts, Vegetable Pathologist, University of Maryland;

Some of the applied research results of trials on management of vegetable diseases that were conducted in Maryland and Delaware, are now available online. These results include evaluations of 1) foliar fungicides for gummy stem blight and anthracnose on watermelon, 2) cultivars and foliar fungicides for downy mildew of processing cucumber, 3) impact of cultivar selection on Phytophthora blight on watermelon, and 4) drip vs. foliar application of fungicide for managing Phytohthora blight on watermelon. The compiled research reports are available at
Click on 2018 Season – Applied Research Results on Vegetable Crop Disease Management.


Vegetable Crop Insect Scouting

David Owens, Extension Entomologist; 

Scout peas for aphids. We have seen unusually high numbers of aphids in other crops. Paraphrasing from my predecessor: “On small plants, sample 10 plants in 10 locations, on larger plants, 10 sweeps…5-10 aphids per plant or 50 per sweep is a good action threshold for aphids in peas.”

Some of the first melon transplants are going out soon. Be advised that striped cucumber beetle may be active earlier this year than in recent years past due to the unusually warm weather.

Insecticide Trial Results for Vegetable and Agronomic Crops

David Owens, Extension Entomologist, and Bill Cissel, Extension Agent – Integrated Pest Management;

Summaries of last season’s insecticide trials in peas, sweet corn, watermelon, field corn, soybean, and wheat can be viewed at

Results from 2018 Vegetable Variety Trials Available Online

Emmalea Ernest, Associate Scientist – Vegetable Crops;

The Extension Vegetable and Fruit Program conducted several variety trials in 2018. Crops that we evaluated for yield and quality characteristics were: seedless watermelon, mini seedless watermelon, baby lima beans, peas, beets and snap beans. Reports on these trials are now available online at:

Vegetable Insect Updates

David Owens, Extension Entomologist, and Bill Cissel, Extension Agent – Integrated Pest Management;

Sweet Corn
Bill Cissel and David Owens
Corn earworm populations are higher than last week, and yesterday’s trap capture was higher in all but one location from Monday’s trap capture. At the research station, our trap captures increased significantly in the last two nights. Blacklight trap captures are also increasing. Focus more on the state-wide trends. Monday trap capture can be found at ( As a reminder, what is reported on the web is on a per night basis, the table below is cumulative over Monday, Tuesday, and Wednesday night. These trap captures correspond to a 3-day spray schedule, though some states recommend a 2-day schedule.

Joe Ingerson-Mahar at Rutgers in New Jersey recently wrote about the need for product rotation, citing pyrethroid resistance monitoring Virginia has been doing ( We have been using Virginia’s vial testing method and have seen similar results, near 30% of tested moths can fly after 24 hour exposure. Does this mean pyrethroids are ineffective? No, but it does highlight the need for chemistry rotation. We also do not know how vial tests translate to sweet corn efficacy where we apply products several times in a narrow time frame.

Trap Location BLT – CEW Pheromone CEW
3 nights total catch
Dover 2 17
Harrington 1 28
Milford 2 41
Rising Sun 0 6
Wyoming 4 65
Bridgeville 3 63
Concord 3 56
Georgetown 4 32
Greenwood 2
Laurel 2 132
Seaford 5 52

David Owens
Spider mite populations have generally been decreasing, but can still be an issue with the warm weather. High humidity favors fungal pathogens of spider mites, but fungicide sprays can suppress them. Infected mites are going to look crumpled and may even be brown in color and a slightly fuzzy appearance as mite-killing fungi sporulate. Cucumber beetles are very active, as are various members of the rindworm complex. Yellow striped armyworms are present in fields.