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Tips for Broadleaf Weed Control in Pumpkins and Winter Squash

Originally published in ONvegetables in The Grower, April 2017

Even with the use of herbicides, broadleaf weed control in pumpkins and squash can be problematic. Product selection is key but timing and weather conditions are also important to the success or failure of a weed control program. Each of the broadleaf herbicides comes with its own strengths, weaknesses and risks.

As a general rule, the spectrum of weeds controlled can be increased by using tank-mixes. But, for pumpkins and squash, it is wise to limit the tank-mix to two products.  A three-way tank mix is risky from a crop safety standpoint; root damage, stunting, yellowing and/or burning may occur, especially under certain soil conditions.

All of the pre-emergence herbicides require soil moisture. The active ingredient is carried by the soil water into the germinating weed seedlings, causing them to die.  Under dry soil conditions, it is tempting to use overhead irrigation to “activate” the herbicides.  This is an inexact science.  Too much water can quickly move the herbicide band into the zone of the germinating crop roots, causing injury to the pumpkins or squash. Too little water may be insufficient to move the herbicide into the germinating weeds.

It becomes a gamble between loosing crop to herbicide damage or loosing yield to weed competition. Fortunately, both Dual II Magnum and Sandea can be used for early post emergence weed control.  Unfortunately, control of weeds such as lamb’s-quarters and pigweed is less effective with a post emergence spray.

Product: Command 360 ME (clomazone)
Timing:  after seeding but before crop and weed emergence.
Rate: 0.78-1.17 L/ha (0.31-0.47 L/acre) use the low rate on light, sandy loam soils; use the high rate on heavy soils.
Strengths: lamb’s-quarters, nightshades, ragweed, velvetleaf
Weaknesses: pigweeds.
Cautions: very sandy soils and/or certain varieties may be prone to crop injury, see label for details. Also refer to the label for rotational crop restrictions.

Product: Sandea (halosulfuron)
Timing(s): after planting and before soil cracking (direct seeded), before transplanting; do not transplant sooner than 7-days after application, OR post-emergence between the 3-5 true leaf stage or 14-days after transplanting[1]
Rate: 35-70 g/ha (14-28 g/acre). See the product label for specific rate information for direct seeded, transplanted, processing and fresh market pumpkins and squash.
Strengths: pigweeds, lady’s thumb, mustards
Weaknesses: lamb’s-quarters
Cautions: Under adverse growing conditions (dry or excessive moisture, cool weather, etc.) the maturity of the treated crop may be delayed which can influence harvest date, yield, and quality. Under dry soil conditions, apply 3 – 5 cm of sprinkler irrigation to settle the soil after planting and prior to application. Do not make a post emergence application if female blossoms are present on the plant; crop damage may occur to developing fruit.

[1] If using pre-emergence and post emergence applications of Sandea, allow for a minimum of 21 days between the two applications.

Product: Dual II Magnum (s-metolachlor/benxacor)
pre-emergence or at the 1-2 leaf stage (direct seeded crops). Prior to transplanting or within 48 hours after transplanting (transplanted crops).
Rate: 1.15 L/ha (0.47 L/acre)
Strengths: annual grasses, nightshades, pigweeds
Weaknesses: lady’s-thumb, ragweed, velvetleaf
Cautions: risk of crop injury increases with cool and wet conditions. Foliar contact can increase the potential of crop injury. Note: research in Ontario has shown high levels of phytotoxicity when using Dual II Magnum on cucumber crops.  Use on cucumber crops is not recommended.

Cucurbit Disease Update – August 7th, 2014

While downy mildew has still not been confirmed in Ontario, recent report from Michigan indicate that levels are starting to increase in the Saginaw Bay area.  At this time growers should maintain preventative downy mildew fungicide programs, especially for the late harvested crop.

Phytophthora has been identified in a pumpkin field in Norfolk county.  This disease thrives under wet soil conditions.  For more information, see: Phytophthora Blight of Cucurbits – August 7th, 2014.

Angular leaf spot, a bacterial disease, is present in many pumpkin fields at this time.  While this is a sporadic disease of cucurbits in Ontario, under the appropriate weather conditions, it could spread a cause premature defoliation of the crop.  Infections may also predispose the fruit to bacterial soft rot at harvest or in storage.  Copper fungicides, such as Copper 53W and Copper Spray, will help to control the spread of this disease.

Angular Leaf Spot on Pumpkin
Angular Leaf Spot on Pumpkin


Phytophthora Blight of Cucurbits

Phytophthora symptoms include stunting, crown rot and fruit rot depending on the cucurbit species and the time of infection. The crown rot phase is the most destructive. Dark-green, “water soaked” lesions form on the crown or vines, girdling it and causing the entire plant to turn yellow or brown and die. (Figures 1 and 2).

Water-soaked lesion on the stem and necrosis of the crown tissue
Figure 1. Stem Lesion (L) and Crown Rot (R)
Yellowing of the foliar, followed by necrosis and widespread croploss
Figure 2. Foliar Symptoms in Pumpkins (L) and Zucchini (R)

Fruit infections begin as a large water-soaked lesion. Under humid conditions, a thin layer of white spores develop on the surface of the fruit. These spores resemble a fine dusting of powdered sugar (Figure 3). In pumpkins, the lesions frequently are circular in shape, while in cucumbers the spores often cover the entire fruit.

White spores on the fruit surface of zuchinni and pumpkin.
Figure 3. Fruit Rot in Zucchini (L) and Pumpkin (R)

Phytophthora is a water mould. It thrives under wet soil conditions caused by excessive rainfall, poor drainage or even compaction. Fields with short rotations are at high risk of developing phytophthora. The spores survive in the soil for long periods of time. Other host crops include the solanaceous crops (peppers, eggplant, tomatoes and potatoes.)

Products registered for phytophthora in cucurbits include Zampro (ametoctradin/dimethomorph) and Presidio (fluopicolide)[1]. Apply fungicides before disease symptoms appear, when the risk of infection is high based on local soil and weather conditions.

In Michigan, phytophthora spores have been found in irrigation ponds and streams. See: The Potential for Spread of Phytophthora Blight of Cucurbits and Peppers in Irrigation Water

The most effective way to manage this disease is through early identification and crop rotation. While a 3-yar rotation is normally sufficient for most cucurbit diseases, fields infested with phytophthora may require a longer (4-5 year) rotation. If you suspect phytopthora in a cucurbit crop field, contact OMAFRA or your local agri-business.

[1] Suppression only. Refer to the product label for rotational crop restrictions.