Weed Management in Cucurbits
Weed management in Cucurbitaceae crops (pumpkins, squash, cucumbers, and melons) is difficult because the crops are low growing, may not close canopy, are relatively noncompetitive, the vining habit does not allow tillage or hoeing, and registered herbicides do not control all problem weeds. We will discuss two long-term studies evaluating different approaches for weed management in Cucurbitaceae. The first study evaluated new nonlabeled and older registered herbicides for pumpkin. Our objective was to expand the range of herbicides labeled for pumpkin and gain more experience with currently registered herbicides. Pumpkin weed management has traditionally relied on preemergent applications of Strategy (a prepackaged mixture of ethalfluralin and clomazone) and Sandea (halosulfuron). Strategy does not adequate control Amaranthus species (waterhemps and pigweeds). Sandea controls Amaranthus species but development of herbicide resistant weed biotypes is a problem. Waterhemp species in pumpkin production areas have developed resistance to ALS-inhibiting herbicides including Sandea and are becoming difficult to control in many Illinois pumpkin fields. Dual Magnum (s-metolachlor) was labeled in 2007 for use in pumpkin production. It will control Amaranthus species and nightshade. In our most recent field studies (2007 to 2009), pumpkin injury was dependent on the time of rating and the herbicide treatment. On July 30 before application of postemergence directed herbicides, Dual Magnum + Reflex (at 0.50 lb/acre) caused more pumpkin injury than any other treatment. Using a lower rate of Reflex (0.25 lb/acre) with Strategy or Dual Magnum did not cause more injury than Strategy or Dual Magnum alone. Compared to the control, Strategy (low rate) + Sandea, Outlook, and Dual Magnum at either rate did not injure pumpkins. On August 8 and 15, Aim (carfentrazone) caused contact burn on pumpkin leaves and pumpkin plants were still injured by the high rate of Reflex applied preemergence. Our directed postemergence (DPost) treatments were not done with a shielded sprayer; illustrating the importance of not allowing Aim to contact crop plants. By the August 15 rating, the pumpkin canopy closed, improving weed control in most treatments. The DPost treatments with either Aim or Reflex and the Dual Magnum + Reflex (at 0.5 lb/ acre) preemergent treatment provided 90% or greater weed control. The combination of either Strategy / Aim or Dual Magnum/ Aim might provide season-long weed control for nightshade and ALS-resistant waterhemp species. Our second long-term study has been evaluating mustard green manure crops as biofumigants before pumpkins and pickles. Mustards have a group of sulfur containing biochemicals called glucosinolates which are defensive compounds. When mustard tissue is damaged by insect feeding or in our studies mowing and tillage, the glucosinolates degrade into isothiocyanates and other compounds that can kill weed seedlings and many soil organisms. Mustard biofumigants could allow pumpkin and cucumber growers to shift to all postemergence herbicide management programs and avoid development of resistant weed biotypes. The effectiveness of mustard biofumigants depended on weed species (grasses and pigweeds better than velvetleaf), incorporation timing, and the specific cultivar. Ida Gold mustard (Sinapis alba, L.A. Hearne Seeds) and Red Giant mustard (Brassica juncea Integlifolia Group, Seeds of Change) had better weed suppression and higher total glucosinolate levels than other cultivars. Ida Gold is sold as a high-glucosinolate containing cultivar for biofumigation. Planting pumpkins and pickles immediately after incorporation of mustard shoots suppressed weeds longer on a Drummer silty clay loam without crop injury compared to delaying planting for seven days after incorporation. In New York, on a low organic matter sandy loam soil, the pumpkins were injured by incorporating mustard. Research continues to refine the system and obtain more consistent weed control without crop injury.