Soil, Crop and Pest Management

Managing Insects and Diseases in Pumpkin and Winter Squash

Craig Hollingsworth, Department of Entomology
John Howell, Department of Plant and Soil Sciences
Robert Wick, Department of Microbiology

Introduction

While winter squash, summer squash, cucumbers, melons, pumpkins and gourds are all closely related, their tolerance to pest damage varies widely. Pumpkins and winter squash are among the most resistant of the cucurbits to certain disease and insect damage. This fact sheet discusses management options for winter squash (referred to here as squash) and pumpkin.

Two critical factors in squash or pumpkin production are nutrient management and weed management. When nutrient levels are too low, the crop will not reach its yield potential. However excessive nitrogen results in excessive foliar growth, reduced bloom and fruit set, and may encourage disease and insect pests. Determine appropriate fertilizer rates before planting using a standard soil test, based on expected crop uptake A pre-sidedress nitrogen test (PSNT) is also recommended. Consult Soil Basics V: Topdressing and Sidedressing Nitrogen, UMass Extension Fact Sheet VegSF11-98, and the current New England Vegetable Management Guide for additional information.

Weed management strongly influences yield and quality of squash and pumpkin, and also affects success of treatments for disease and insects. Herbicides labeled for squash and pumpkin may not provide complete control in all conditions. It is common to supplement herbicides with cultivation and some hand weeding. Squash and pumpkin should not be planted in fields where weed pressure is high. Information on herbicide selection as well as non-chemical weed management options can be found in the current New England Vegetable Management Guide.

General Scouting Considerations

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Pumpkin and squash are scouted for two reasons:

1) to assess the insect and disease situation of the whole field, and 2) to detect local problems which may require immediate treatment. When assessing the whole field, samples should be selected so that a representation of the whole field is provided, but when looking for specific problems ( areas where problems are mostly likely to occur (e.g. wet areas, field borders) should also be examined.

Scouting procedures will vary through the season, as the presence of primary pests changes. In general, each week examine 50 leaves (including older and younger leaves) throughout the field along a line or in a V-, X- or W-shaped pattern. As vines fill in, it is often necessary to use tractor wheel rows to access plants in the middle of the field. It is important to be alert for evidence of pests anywhere in the field in addition to evidence on sampled plants.

Keeping records of scouting results will improve your pest management. Records help you remember where problems exist so that you can evaluate and adjust your IPM practices appropriately.

Insects

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Insect management recommendations are provided in the current New England Vegetable Management Guide. Squash and pumpkins are dependent on insects for pollination: insecticides should not be applied during maximum flowering and fruit set. If insecticides must be used, choose materials least toxic to bees (consult the New England Vegetable Management Guide) and apply in the evening, when bees are not active in the field.

Cucumber Beetles

 Striped cucumber beetle is most important early in the season on young transplants and emerging seedlings. Because winter squash and pumpkin are not susceptible to bacterial wilt (primarily a cucumber and melon disease), they can sustain significant defoliation without a loss in yield. In mid-summer, spotted cucumber beetles and western corn rootworm beetles feed on blossoms. Damage to fruit has also been reported.

Emergence to four-leaf stage

Sampling: This is a critical time for plant establishment and cucumber beetle numbers can increase quickly. Scout fields twice weekly, if possible. Inspect 25 plants, walking across field in a line or a V-shaped pattern and selecting plants without bias. Count the number of beetles and estimate the amount of defoliation per plant. In addition, check along field borders for defoliation. Calculate the average number of beetles per plant.

Thresholds: Early in the season, pumpkins can tolerate up to 80% leaf loss without affecting yields, but squash is more sensitive. Consider spot treatments where "hot spots" can be defined.

Pumpkins: 5 or more beetles per plant

Squash: 2 or more beetles per plant

Five-leaf stage to harvest

Sampling: As plants grow, it becomes difficult to count numbers of beetles per plant. Plants should be assessed for feeding damage to foliage or blossoms. Late in the season, assess fruit for feeding damage.

Thresholds: If damage is apparent on foliage throughout the field or if blossoms are heavily infested, a treatment may be necessary. After fruit set, a treatment may be needed if there is damage to fruit or to pumpkin handles.

Squash Bugs

Squash bugs are generally not a problem in commercial plantings but may be found in squash and pumpkin fields late in the season. Adults and nymphs may feed on leaves and stems, leaving scorched-looking damage. Late in the season, adults pierce fruit with their mouth parts. If populations are high, this can cause cosmetic injury to the skin of pumpkins. Look for orange-reddish eggs laid in clusters on upper and lower leaf surfaces. Aggregations of nymphs may be found under leaves and fruit, especially in contact with the soil. Adults can be found at the base of plants near the soil line. Specific sampling methods and thresholds have not been developed for squash bugs.

Aphids

Several species of aphids may occur in pumpkin and squash. Rarely, high densities of aphids can cause plants to wilt, curl their leaves, turn brown and die. Such damage does not occur until after plants have produced runners. Even at low levels, aphids can spread virus among plants. However, insecticides are of questionable value in controlling viruses, as they may encourage movement of aphids and increase the spread of virus. Application of insecticides may be necessary to prevent excessive damage from aphid feeding.

Sampling : Examine the undersides of 50 leaves from plants selected across the field. Do not count all of the aphids, but determine whether or not each leaf has more than ten aphids on it.

Threshold: If more than 20 percent of leaves examined (10 of 50 leaves) have ten or more aphids present, treatment may be needed. Also consider the presence of natural enemies. Treatment may not be needed if aphids are parasitized (look for aphid mummies), or if aphids are infected with fungi.

Diseases

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Pumpkins and squash are susceptible to a wide range of diseases which can affect yield, quality and storage of the crop. Crop rotation with non-host crops will reduce survival of plant pathogens and provide opportunities for improved weed control. Disease-resistant cultivars, and disease-free and treated seed should be used whenever possible. Land should be rotated out of cucurbits for a minimum of two years, or more where specific diseases warrant it.

Scouting for Diseases

Fields can be scouted for insects and diseases at the same time. Examine 50 leaves as described above. The table on the following page indicates the timing of appearance and general symptoms of the principal diseases in squash and pumpkin.

Chemical Control for Diseases

Chemical recommendations are provided in the current New England Vegetable Management Guide. The recommended spray program for 1998 is:

1. Where black rot (gummy stem blight) is of concern, apply Bravo* as fruit reach 4 inches.
2. When powdery mildew has been detected, apply Benlate* or Topsin M*, plus Bravo* (or equivalent) at highest labeled rates for both products.
3. After 14 days, apply Reach*, or Bayleton*plus Bravo* (or equivalent) at highest labeled rates.
4. After another 14 days, apply Bravo* (or equivalent) and continue regular application until 2 to 4 weeks before end of harvest. Add Benlate* or Topsin M* if black rot is of concern. Add Ridomil*/Bravo* or Aliette* plus maneb if downy mildew occurs.

Fungicide Resistance

Fungicide resistance is a concern for black rot (gummy stem blight), downy mildew, and especially for powdery mildew. Resistant strains of powdery mildew have been found to systemic fungicides, including triadimefon (Bayleton* and Reach*) and benomyl (Benlate*). Benomyl-resistant strains are also resistant to thiophanate methyl (e.g. Topsin M*). The following management strategies should be used to delay fungicide resistance:

1. Scout fields regularly to determine when the disease is first present.
2. Do not apply systemic fungicides unless the disease is present.
3. Apply systemic fungicides when disease first appears
4. Never apply systemic fungicides alone. Combine them with a contact fungicide (e.g. chlorothalonil/ Bravo*).
5. Alternate between systemic fungicides with different active ingredients (e.g. Bayleton* or Reach* followed by Benlate* or Topsin*) when possible.
6. Maximize spray coverage on leaf undersurfaces:
• Drive slowly (2 mph or less)
• Use high sprayer pressure (80 psi)
• Use high volume (75 gpa)
• Use closely-spaced nozzles (10 inches)
• Direct sprays at an angle to canopy (15^o)
• Check spray coverage with water sensitive cards

* trade name

This fact sheet is based on our experience in developing an IPM program for winter squash in Western Massachusetts and from current recommendations from other universities. We gratefully acknowledge the assistance received from Cornell University Extension, especially T.A. Zitter, M. Hoffman, M.T. McGrath, C.H. Petzoldt, A.J. Seaman and L.H. Pederson, for providing much of the information presented here.

This information is for educational purposes. Recommendations contained here are based on the best available knowledge at the time of printing. Any reference to commercial products or trade names is for information only, and no endorsement or approval is intended. Always read the label before using any pesticide. The label is the legal document for product use. Disregard any information in this fact sheet that is in conflict with the label.

University of Massachusetts Extension

Printed June 1998

VegSF6-98