How to Manage Pests

UC Pest Management Guidelines

Crops in the cucurbit family include melons, watermelon, squash, pumpkin, luffa, gourd, and cucumber. Each crop has its own growth characteristics, which in turn influence weed management options. Cucurbits are most commonly grown in the warmest areas of the state, such as the San Joaquin Valley, the Sacramento Valley and the low desert valleys. Weeds can cause yield reductions, especially when crops are not grown during their preferred season. In addition, weeds interfere with harvest by making fruit difficult to find. The vigorous growth of many cucurbits makes integrated weed management feasible and reduces the need for herbicides. In any case, an integrated approach is necessary because of the limited availability of registered, effective, selective herbicides.

Vigorous, rapid growth of some cucurbits (melons, watermelon, and squash) during warm seasons makes them very competitive with the weeds a single cultivation may be all that is needed for weed control. Other cucurbits such as cucumbers grow less vigorously and require additional weed control measures. Most cucurbit crops in California are grown in the open, but to produce fruit out of season the crop is grown in modified climates or under techniques such as hothouses, tunnels, plastic mulches, mid-bed trenches, and row covers. When cucurbits are grown out of season, they grow more slowly and are less competitive against weeds, requiring a more diligent weed control program for optimum yield and quality. In these situations, cultivation is not possible and hand hoeing or preplant fumigants is necessary. Crop rotation is also a useful weed management tool because it allows different control measures to be used in the various cropping systems, thus avoiding the increase of specific weed populations.

Herbicides, combined with good cultural practices, control many weed pests of cucurbits. Generally, when a cucurbit crop is grown using climate modification techniques, soil fumigants are used before planting. Otherwise herbicides are applied before planting and mechanically mixed in the soil (preplant incorporated) or after planting and incorporated with irrigation (preemergent). Other herbicides are applied after planting to emerged weeds and are referred to as postemergent herbicides. Preemergent herbicides can also be applied during the layby period to keep the crop weed-free until harvest.

The choice of herbicide depends upon the weed species expected to occur and plantback restrictions. Plantback restrictions must be carefully considered because herbicide soil residues can limit the growth of sensitive rotational crops. Read labels closely for information regarding registration, use, and plantback restrictions.

MONITORING

Monitor the fields and keep records of the weed species that occur in each field during the period of the year when the crop will be grown, especially at planting time. Not only are these records valuable in planning weed management strategies, but also they help track the occurrence of hard-to-control weeds. At harvest, keep records of specific locations in the field where weeds are producing seed. Weed patches are relatively stable within a field and spot treatments to these weed clusters may improve weed control in future crops. Avoid fields that have high populations of certain weeds such as common purslane, field bindweed, or nutsedge, as these weeds are not adequately controlled by currently available weed management methods.

WEED MANAGEMENT BEFORE PLANTING

To prevent the buildup of weed seed in the soil, cultivate weeds before they set seed in rotation crops. After harvest of the rotation crop, clean cultivate the field, plant a green manure crop, or use an herbicide to prevent weed infestations.

Just before planting cucurbits, preirrigate the field to germinate weed seeds and cultivate or use a nonselective herbicide such as paraquat or glyphosate (Roundup) to destroy them. (Glyphosate can be particularly helpful in controlling perennial weeds before the crop is planted.) Carrying out this operation as close to planting time as possible ensures that soil temperature and climatic conditions are similar to those that will occur during the crop germination period, thus maximizing the number of weeds controlled. Cultivate as shallowly as possible in order not to bring up dormant weed seed from deeper soil layers.

Preplant fumigants. Soil fumigants are necessary to prevent severe yield loss from weeds that are favored by the warm conditions created when climate modifications such as a clear soil mulch, tunnel, or mid-bed trench are used. Metam sodium is used as a soil fumigant before planting a cucurbit crop. It is used primarily to control soilborne diseases and nematodes, but it also controls weeds. Weed control results have sometimes been inconsistent with metam sodium; be sure to apply it to well-prepared, moist seedbeds that are free of clods.

Herbicides. Bensulide (Prefar) is a narrow range, persistent herbicide that controls some small-seeded annual grasses, such as annual bluegrass, barnyardgrass, crabgrass, and some broadleaf weeds, such as pigweed and purslane. It can be mechanically incorporated shallowly (1 to 2 inches) before planting, or applied after planting under sprinkler irrigation. Consider the weed history of a field to determine the potential value of a bensulide application. Corn, sudangrass, and sorghum are sensitive to soil residues of bensulide following a cucurbit crop; see label for specific restrictions.

WEED MANAGEMENT AT PLANTING

Plant or transplant cucurbits into uniform beds utilizing a precision planting system that will promote a uniform crop and allow cultivation close to the seed line. This reduces the need for hand hoeing and lowers weed control costs.

Planting into a preirrigated field with dry surface soil is feasible for all cucurbits but is most effective for large-seeded ones such as melons, watermelon, and squash. This technique involves planting the seed into moist soil below a dry soil layer. To use mulch planting, first preirrigate the field, and then shallowly cultivate 2 to 4 inches deep when the surface dries. Cultivation kills any emerged weeds and creates a dry barrier (mulch) over moist soil into which the crop is sown. The field is not irrigated again until the crop reaches the third to fourth leaf stage. Mulch planting can fail and herbicide treatments may be required if it rains after sowing, if field conditions (e.g., soil type, fertility) are irregular, or if the mulch layer is too deep and irrigation is required to germinate the crop.

WEED MANAGEMENT AFTER PLANTING

After planting but before the crop or weeds emerge, bensulide can be applied and incorporated with sprinkler irrigation or through chemigation to control small-seeded annual grasses and some broadleaf weeds. Paraquat and glyphosate (Roundup Ultra) can be used to control emerged weeds after planting but before crop emergence. Take care, as emerged cucurbit plants will be killed if contacted by these herbicides.

After the crop emerges, sethoxydim (Poast) can be used to control seedlings of some annual and perennial grasses. Its effectiveness is reduced when grasses are under moisture stress. Later growth stages of annual grasses are more difficult to control. Follow label instructions regarding the use of adjuvants with these herbicides. Sethoxydim will not control annual bluegrass and it varies in its ability to control particular grass species. For effective control of perennial grasses (bermudagrass and johnsongrass), two applications will be required.

During the growing season cultivation practices vary depending upon the crop grown, the season it is grown in, and the use of climate modification techniques. Close cultivation is only possible before runners (vines) are produced. Hand hoeing is often used to supplement machine cultivation and thin the crop to the required density. Late-season hand hoeing can help reduce weed seed but almost always results in some yield loss.

Layby. During cooler seasons or for crops that have a long growing season, a layby herbicide can be beneficial. Trifluralin, DCPA (Dacthal), and ethalfuralin (Curbit) can be used after thinning or during the layby period to control late emerging grasses and annual broadleaf weeds. They are applied as a directed spray to the soil surface when the crop has four to five leaves, taking care not to contact the crop foliage. Trifluralin requires mechanical incorporation and ethalfuralin is most effective when it is incorporated with a sectioned rolling cultivator within 1 to 2 hours after application. DCPA does not require mechanical incorporation. None of these herbicides will control emerged weeds; they are only effective on germinating seed. Their main benefit is to keep the weed populations low to facilitate harvest. Some carryover can occur under certain conditions, creating a plantback problem. Consult the herbicide label before application.

PUBLICATION

UC IPM Pest Management Guidelines: Cucurbits
UC ANR Publication 3445
Weeds
W. T. Lanini, Plant Sciences/Weed Science, UC Davis
Acknowledgment for contributions to Weeds:
C. E. Bell, UC Cooperative Extension, San Diego County

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