There are a limited number of herbicides registered for melon production in California and Arizona, two of the major production areas for cantaloupes and honeydews. Shallow cultivation and nonselective herbicides with no soil activity can be effective if used after bed formation and before planting. Both contact (paraquat [Gramoxone]) and systemic (glyphosate [Roundup]) herbicides can be used. Curbit (ethafluralin) can be effective, but should be used with caution, as crop injury will occur if it is concentrated around the germinating melon seed by water or cultivation.

While used mainly for post emergence nutsedge control, halosulfuron (Sandea) is currently registered on melons for both pre and post emergence broadleaf weed control, including pigweed and purslane. It is safe to use on most melon types after they have three leaves and before blooming. Carryover in the soil is a consideration if sensitive crops are to follow (8 months for tomatoes, > 12 months for many leafy greens and brassica crops). As with many herbicides, efficacy is improved with sprinkler incorporation.One challenge with Sandea, however, is the preharvest interval required on the label of 57 days (57-day PHI). Post emergence applications likely will be more effective because of greater weed emergence after the first irrigation (in furrow irrigated fields), but may be too late in the season since melons are usually harvested less than 90 days after emergence.

Unfortunately, halosulfuron does not control grasses, and therefore tank mixtures with clethodim (Select Max) herbicides may improve overall weed control in melons. Grass weed control is improved with the addition of adjuvants, and crop oil concentrate at 1% v/v is recommended on the label. However, crop oil concentrates are not recommended with halosulfuron because they increase the chance for phytotoxicity on the crop. Melon Board sponsored research in 2016 showed that a tank mix with Sandea and clethodim was effective and safe on the crop when no adjuvants were used, and that the addition of surfactants could significantly increase crop phyto.

To evaluate this, two trials were performed in 2017 to evaluate both the timing and efficacy of POST emergence combinations of halosulfuron (Sandea) and clethodim (Select Max) herbicides on direct seeded melons in central California. Trial locations were in commercial honeydew and cantaloupe fields. Sandea was applied at 1 oz/A, and Clethodim 2E (26.4% ai) at 8 oz per acre. No adjuvants were used for any of the treatments. POST treatments were made at about 4 true leaves in the honeydew field, and 2 true leaves in the cantaloupes. Both applications were sprinkler incorporated within 24 hours. At 28 days after emergence, POST treatments were applied as a directed spray to the outside and shoulder of the beds to minimize contact with foliage, and as an over-the-top application across the top of the bed. The 28-day application was not sprinkler incorporated.

POST treatments caused no significant crop injury but significantly reduced weed pressure. The 2–4 leaf treatments, which were sprinkler incorporated, provided best overall weed control at all test locations, 81% weed control in the cantaloupe trial (Figure 1). Main weeds at this location were field bindweed, pigweed, purslane, nightshade, puncture vine, and volunteer wheat. There were very few weeds at the honeydew location, most likely because Roundup Ready cotton was the previous crop. No significant differences were observed in crop injury between the timing of herbicide application for either melon type, but slightly more herbicide injury was noted in the treatments containing clethodim (Figure 2). Fruit production for both melon types was similar across all treatments and there was no significant impact on yield.

Results showed that Sandea applied at 2–4 leaves to be safe and effective when incorporated with sprinklers on both honeydew and cantaloupes, providing good weed suppression across a spectrum of broadleaf weeds. The addition of clethodim did not improve weed control, since there were few grassy weeds within the test area, but it did slightly increase crop injury with the over-the-top application at 28 days.

 

Scott Stoddard is a Farm Advisor for the UC Cooperative Extension Merced and Madera counties. csstoddard@ucanr.edu