Phytophthora Blight in Vine Crops

Heavy rains in some areas may set up conditions for Phytophthora blight in watermelons, pumpkins, cucumbers, squash, and other cucurbits. The following is more information.

In the past three weeks I have received many reports of Phytophthora blight in watermelon fields on Delmarva. This disease appears to be especially common this year on watermelons. We have had a few periods during the summer where we had high volume rain events. The threshold that usually triggers disease development is 2 inches of rain that falls over a short enough period of time to pool in the field. If soil is saturated for 5 to 6 hours, the zoospores are released and a new infection cycle will begin. Optimum temperature for spread is 28C (82°F). There are several reasons that disease might be especially severe this year. In addition to high volume rain events, soil compaction may be greater this year because growers had to work in fields during June when soil remained wet from frequent rains. Soil compaction would slow drainage and increase the length of soil saturation.

Management practices for this disease must begin prior to planting. Remove infected debris from fields, including, where possible, diseased fruit. Cultural practices for management of Phytophthora blight are to improve soil drainage through tillage, use raised beds and reduce soil compaction. Alternate hosts include beans (snap and lima), cucurbits (pumpkin, melons, cucumbers, etc.), eggplants and tomatoes.

Fumigants such as K-pam. Vapam and Telone will reduce plant death, but fumigation should not be used as a stand-alone practice. Fumigants and fungicides, used in an overall disease management program, which includes cultural practices, is the best approach.
The fungicides available for Phytophthora blight control are, at best, suppressants of disease. Forum, Gavel, Tanos, Presidio, Revus and Ranman are labeled. Bob Mulrooney wrote a good overview of treatments in a Weekly Crop Update article a few weeks ago

Information from Kate Everts, Vegetable Pathologist, University of Delaware and University of Maryland

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Squash Bugs Heavy Again Early This Year

Squash bugs are heavy again this year and are attacking squash and melons at this time. The following is more information.

Squash Bugs in Cucurbits (including melons)

At this same time last season, Jerry Brust from the University of Maryland did a nice article for the Weekly Crop Update titled Squash Bugs in Pumpkins. Be sure to follow this link for good pictures of the adult, egg masses and damage. It appears that this season we are again seeing squash bugs in cucurbits. Some consultants are reporting what they feel are higher levels of squash bugs in watermelon fields compared to past seasons. In past years we have seen this pest cause problems in squash and pumpkins and it appears that they could be causing damage in some watermelon fields.

Ruth Hazzard of the University of Massachusetts Vegetable Extension Program has written a fact sheet about squash bugs in vine crops which includes information on which crops are affected, plant damage and the life cycle of the squash bug. Be sure to follow this link for more detailed information:

The following are a few key points from this fact sheet:

● “Squash bugs are most attracted to Hubbard squash, summer squash, pumpkins, watermelons, muskmelons, cucumbers, and butternut squash in decreasing order. Because of low attraction and low survival rate, squash bugs do not usually become a pest on cucumber, watermelon, butternut squash and muskmelon.”

● “Squash bug feeding can cause wilting in leaves, stems, and vines that are beyond the feeding site. The injury may appear as light-colored areas that later turn brown and die, symptoms that resemble bacterial wilt.” It is important to remember, there are a number of factors that can cause wilting in cucurbits – including a number of diseases – so a correct diagnosis is critical.

● Although squash bugs may also vector Cucurbit Yellow Vine Disease (CYVD) caused by the bacterium Serratia marcescens, this disease has not been detected in our region. There is a link in this fact sheet to an article from the University of Connecticut on cucurbit yellow vine disease

● “In late summer and fall, large nymphs and new squash bug adults can also damage the fruit of fall vine crops.”

Squash bug and eggs. Photo by Jerry Brust, University of Maryland.

Squash bugs at the base of wilted pumpkin plant under plastic mulch. Photo by Jerry Brust, University of Maryland.

Information from Joanne Whalen, Extension IPM Specialist, UD

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Weather Conditions and Transplants

This past week has been challenging and growers have been trying to set transplants between the rains. The following is an article on weather conditions and transplants.

Weather conditions currently are not favorable for the growth of warm season vegetable transplants (watermelons, cantaloupes, tomatoes, peppers, eggplants, cucumbers, squash). We had some unusually warm weather from April 25-28 with average air temperatures in the 70’s that allowed early plantings to go in on plastic mulch with promise of good establishment. From April 29 onward, average air temperatures have been mostly in the 50’s and we have had rainfall every day from May 1 to May 7. This weather is expected to continue until Sunday. Next week promises some sun but temperatures will still be moderate.

Warm season vegetable transplants vary in their ability to withstand sub-optimal conditions depending on how well they have been hardened off and their inherent ability to withstand stress. Tomatoes, cucumbers, and squash are better able to handle early season stresses than cantaloupes, watermelons, or peppers. When temperatures are cool and soils are wet, growth is minimal in these crops. We often see problems, especially the first few days when sunny weather returns, with plants wilting. This is because root systems have not established or are not functioning well. Root growth is slowed in cold soils and low oxygen in water soaked soils will also limit root growth. Average soil temperatures need to be 65°F or higher under the plastic and average air temperatures should also be above 65°F (ideally above 70°F) for good establishment of these crops. Seed corn maggots and root diseases such as Pythium can further stress transplants and reduce stands.

The following are some considerations when transplanting warm season vegetables under sub-optimal conditions:

· Make sure transplants have well developed root systems (transplants easily pull from trays and have full root balls); do not rush transplants into the field.

· Make sure transplants have been hardened off well by exposing them to outside conditions, eliminating fertilizer, and controlling watering well ahead of planting.

· In seedless watermelon systems, time production of pollenizer transplants so that they coincide well with the seedless transplants. Pollenizers are often planted a number of days after seedless because they emerge quicker. However, pollenizer root balls may not be well formed compared to the seedless transplants and they can suffer excessive losses in the field when planted in stressful conditions. The opposite can also be true if pollenizers are ready but the seedless plants do not have good root balls.

· Leggy plants will be a problem in stressful conditions and should not be used if at all possible. Leggy plants are more susceptible to damage in transplanting and wind damage after planting thus subjecting them to additional stress. Unfortunately, cloudy overcast weather often leads to stretch in transplants.

· Transplants should be planted at the proper depth. This is particularly critical for watermelons and cantaloupes. There should be enough soil to cover the root ball of these crops but they should not be planted so deep so that the stem is covered. Deep planting in cold wet soils will result in additional stress on melons. Watermelons and cantaloupes should not be set deeper even if they are leggy.

· Extra care should be taken during transplanting during stressful periods to reduce injury to plants, particularly to root balls. Damage to roots will reduce establishment success especially in melons, cucumbers, and squash. Train planting crews so that they do minimal damage to transplants.

· Target lighter sandy soils that are well drained for planting in cold and wet periods. Leave out fields or sections of fields with low areas or areas that are excessively wet and plant them when more favorable weather conditions return.

· If plants will hold, it is best to wait until more favorable weather returns. Often there is no earliness gained by planting in the stressful period; or gains are negated by stand losses and the need to replant areas.

Gordon Johnson, Extension Agriculture Agent, UD, Kent County

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Fusarium Crown Rot, Root Rot, and Wilt of Cucurbits

Recently, Fusarium crown rot was diagnosed on a commercial planting of summer squash. Different Fusarium species can attack watermelons, cantaloupes, cucumbers, summer squash, winter squash, and pumpkins. This is a very difficult disease to control with rotation and resistant varieties where available being the best options. The following is some information on this fungus.

Fusarium wilt of melon (cantaloupes and muskmelons) is caused by a seed- and soilborne fungus that is specific to melon (Fusarium oxysporum f. sp. melonis). On young seedlings, a hypocotyl rot and damping-off may occur. In older plants, there is marginal yellowing progressing to a general yellowing of the older leaves, and wilting of one or more runners. In some cases, sudden collapse occurs without any yellowing of the foliage. On stems near the crown of the plant, a linear, necrotic lesion may develop, extending up the plant and usually on one side of the vine. One runner on a plant may wilt and collapse, with the rest of the runners remaining healthy. A gummy, red exudate may ooze from these lesions, but this may also be caused by gummy stem blight and insect injury. Vascular discoloration should be evident and is very diagnostic. Mature plants often wilt severely (collapse) late in the season because of the fruit load stress. Disease severity is maximum at soil temperatures of 64-77°F and declines dramatically above 86°F. At high soil temperatures, plants become infected but may not wilt; rather they develop severe stunting. As with most Fusarium wilts, the following apply: 1) low soil moisture favors the pathogen and accentuates the wilting symptom; 2) high nitrogen, especially NH4-nitrogen, and light, sandy, slightly acidic soils (pH 5-5.5) favor disease development; and 3) liming the soil to increase the pH to 6.5-7.0 decreases wilt severity. Crop rotation is generally not totally effective because chlamydospores survive so long in the soil and the pathogen can survive in or on the roots of symptomless carrier plants. Soil fumigation with a broad-spectrum biocide provides good initial control, but recolonization of the soil occurs very quickly. Liming the soil to pH 6.0-7.0, as well as reducing nitrogen levels in the soil, significantly reduces wilt. The most effective and practical means of controlling Fusarium wilt of melon is through the use of resistant varieties. Work to develop resistant varieties began as soon as the disease was described, and now resistance genes conferring resistance to races 0, 1, and 2 have been incorporated into U.S. commercial varieties and hybrids. One such hybrid that performs well in Delaware is the variety Athena.

Fusarium crown and foot rot of squash and pumpkin is caused by Fusarium solani f. sp. cucurbitae. The pathogen exhibits host specificity for all cucurbits. Only certain squashes and pumpkins seem to be affected. The organism is also thought to exist as races. While its distribution in the United States is limited, it still remains a concern for cucurbit growers. We have seen the crown and foot rot phase of disease sporadically in Delaware and the fruit rot phase has been found in pumpkins in the state. Symptoms. The first symptom usually noticed in the field is wilting of the leaves. Within several days, the entire plant may wilt and die. If the soil is removed from around the base of the plant, a very distinct necrotic rot of the crown and up per portion of the taproot is evident. The rot develops first as a light-colored, water-soaked area which becomes progressively darker. It begins in the cortex of the root, causes cortex tissue to slough off, and eventually destroys all of the tissue except the fibrous vascular strands. Infected plants break off easily about one inch below the soil line. The fungus generally is limited to the crown area of the plant. The main and lower portions of the taproot are not affected, except under extremely wet conditions. Likewise, the stem is not affected, except for the lower inch immediately above the soil line, as seen in experimentally inoculated pumpkin seedlings. Plants showing symptoms develop numerous sporodochia and macroconidia (spores) giving the mycelia a white to pink color on the stem near the ground surface. Fruits are attacked at the fruit-soil interface; the severity of the fruit rot depends on soil moisture and the stage of rind maturity at the time of infection. Because the fungus survives in the soil for only two to three years, a four-year rotation is usually adequate for disease control. Planting fungicide-treated seed also reduces disease initiated from infected seed.

Fusarium wilt of watermelon is caused by the fungus Fusarium oxysporum f. sp. niveum. In the USA, we have two types or “races,” of Fusarium wilt of watermelon. Race 1 is the most common type. Plant breeders have developed many watermelon varieties that are resistant to race 1. Some varieties, like Jubilee; have a low level of resistance, others have a medium level, like Charleston Gray; and some, like Calhoun Gray, have a high level of resistance. This was all well and good for the past 35 years or so. As hybrids with resistance to race 1 were grown more widely and more often, the Fusarium fungus began to mutate. Some mutants turned out to be “super bugs,” able to attack these resistant varieties. These new strains are called race 2. Resistance to race 1 will not prevent melons from getting Fusarium wilt caused by race 2. Race 2 is worse than race 1, because there is no resistance to race 2. So far, race 2 has been found in Texas, Oklahoma, Florida, Maryland, Delaware, and Georgia, but not South Carolina. Most, but not all, diploid (seeded) watermelon varieties on the market today have resistance to race 1. Examples of varieties that have been consistently resistant in field plots in North Carolina, Georgia, Florida, and Maryland are: Crimson Sweet, Jubilee II (not Jubilee), Royal Star, Royal Sweet, Royal Majesty, and Sangria. All triploid (seedless) cultivars are susceptible to race 1 except Seedless Sangria. All commercial watermelon varieties are susceptible to race 2, which is why this disease remains a threat to the watermelon industry. Won’t fumigation control Fusarium wilt? Yes and no. Yes, fumigation will knock Fusarium back, especially if it was not too widespread or severe the previous year. But Fusarium can survive in soil deeper than any fumigant can reach. Of course, the pathogen also survives in nonfumigated soil in alleys between fumigated beds. Fusarium can wait deep in soil (at least 2 feet) or in the alleys until a watermelon root grows that deep or that far, then germinate and infect. So, fumigation can, and often does, help, but it will not eliminate (eradicate) Fusarium altogether. A new promising management tool studied in Maryland is hairy vetch as a winter cover crop. When hairy vetch is disked into soil in the spring, the residue produces ammonia, which is toxic to Fusarium. Like fumigation, hairy vetch will not eliminate Fusarium, but it will knock it back. ‘Cahaba White’ hairy vetch is the recommended variety because it is resistant to root-knot nematode. The vetch can be sprayed with herbicide before disking in the spring.

Information from Cornell University and Clemson University.

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Wilting in Summer Squash, Winter Squash, and Pumpkins

There have been many reports of wilting in summer squash, winter squash, and pumpkins in the county. There are several possible reasons for this problem:

  • Bacterial Wilt. Bacterial wilt is carried by cucumber beetles. There is a great deal of variability of susceptibility to bacterial wilt in squashes and pumpkins. If you are growing a susceptible variety, an agressive cucumber beetle program is necessary if the beetles are present, especially with young plants.
  • Root and crown rots. Pythium damping off and root rot is common as temperatures increase in planting beds, especially in wetter soils. Phytophthora will attack roots and crowns and cause plant collapse. It is found in wetter soils. Both of these organisms are water molds and proliferate in wet conditions. Some Fusarium species, another group of soil borne fungi, can also cause crown rots and wilts in squashes. Specific fungicides used for water molds can be effective on Pythium and Phytophthora. Fusarium must be controlled with rotations.
  • Squash vine borer. The squash vine borer moth emerges in June. The moths fly lay eggs singly on stems; eggs are usually found on the main stem near the base. Moths are active for about one month. Eggs hatch in 9 to 14 days. Larvae enter the stem at the plant base within a few hours after hatching from the eggs. Larvae feed inside the stem for 4 to 6 weeks and cause plant collapse. Control by appling insecticides at the base of the plants from the second week in June through mid July (pyrethroid insecticides, Sevin, and Thionex are used).
  • Squash bugs. High levels of early squash bugs are being seen this year. The following is some information from Jerry Brust, IPM Vegetable Specialist, University of Maryland: Many of the pumpkin fields in the Delmarva area I have visited in the last two weeks were just coming up or had 2-5 leaves on them. I was surprised to see several squash bugs on these small plants. I also found many egg masses on the underside of leaves, usually in the crotch of two veins. In some fields with plastic the squash bugs were feeding below the plastic mulch causing the plants to wilt and eventually die. Normally there are only a few fields that will have squash bugs this early, but just about all of the fields I looked at had enough squash bugs to justify treatment. Growers need to watch for squash bugs on their early pumpkin plants, especially down in the plastic hole. A spray may be needed if the plants are stressed (like they were last week from the intense heat) and there are 2 bugs or 1 egg mass per plant. Insecticides used include several pyrethroids, Sevin, and Thionex.

Gordon Johnson, Extension Agriculture Agent, UD, Kent County

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Nematode Damage Very Evident This Year

Drought conditions often magnify the effects of nematode damage to crops as root systems are damaged and plants have additional water stress. Two problems that have been showing up in Kent county are Root Knot Nematode and Soybean Cyst Nematode. Root Knot Nematode affects soybeans and many vegetable crops. There are a few root knot nematode resistant soybeans and some vegetables such as tomatoes have varieties resistant to root knot. However, resistance is lacking in most soybean varieties and most vegetable crops. Soybean cyst nematode infestations are also on the increase with shifts from race 3 to race 1 being a problem (there is only low level of resistance to race 1 in most soybean varieties). Nematode control is largely achieved through rotation to non-host crops. Some nematicides are available for vegetable crops.

Soybean Cyst Nematodes on Soybean Roots

Soybean Cyst Nematodes on Soybean Roots

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