Phytophthora, films and fumigants

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

In 2009, Michigan producers grew over 80,000 acres of vegetables that are susceptible to Phytophthora capsici, including cucumber, zucchini, summer and winter squash, watermelon, cantaloupe, pumpkin, pepper, eggplant, tomato and succulent bean. The pathogen may overwinter in the soil and persist for many (greater than 10) years. Phytophthora capsici is favored by rain and warm temperatures and spreads readily via water. It has also been found in irrigation ponds and surface water sources. Fumigants, combined with good management practices, can reduce the likelihood of infection occurring in the field.

When considering fumigation, it is important to take several factors into account. In Michigan, fumigation is recommended during the fall as soil temperature will more likely be appropriate for effective fumigation. The soil temperature for fumigation is what allows the fumigant to become a gas. If it is too hot, the movement of the fumigant will be too fast. If it is too cold, the fumigant effects will be limited. It is also important for the soil to be at 50-80 percent field moisture capacity. The soil in the seed bed needs to be aerated; avoid hard-packed clumps and limit the crop residues left on the soil.

In order for the fumigant to work, a moisture seal to lock in the fumigant needs to be present. There are two ways to accomplish this task. A plastic film seal can be applied on the soil surface, or machinery seals are another option. Two different plastic mulches are available to producers: virtually impermeable film (VIF) and low-density polyethylene (LDPE). VIF has a nylon layer that holds the fumigant in the soil better than LDPE, potentially increasing effectiveness. However, sometimes vegetables grown in beds covered in VIF will have phytotoxic injury because the fumigant has not completely off-gassed. Fumigants can be injected into the soil with shank or drip applications.

A large-scale field trial on a commercial farm tested reduced rates of fumigants applied under LDPE and VIF on yellow squash. The trial was established in a field with severe Phytophthora disease pressure, and replicated four times in a randomized complete block design. Treatments included 300 lb Methyl Bromide/Chloropicrin (both 67/33 and 50/50), 230 lb Chloro-Pic 60, 150 lb Midas, and 30 and 60 gal Sectagon K54. Each mulch treatment was paired with an unfumigated/LPDE control. Holes were punched in the plastic and seeds were sown after the appropriate off-gassing period had expired. Data on stand counts and yield were taken and analyzed.

In Trial 1, both formulations of Methyl Bromide/Chloropicrin, Sectagon K54, and Midas had significantly less disease than the paired untreated control. The high rate of Sectagon K54, applied as a broadcast treatment and bedded 14 days later, significantly impacted the vigor of squash plants in all replicates. The low rate of Sectagon K54, applied 14 days before bedding paired with VIF, did not impact plant vigor.

Trial 2 compared two rates of unregistered dimethyl disulfide (DMDS, Paladin™) with two rates of Methyl Bromide/Chloropicrin applied under VIF. DMDS at 40 gal (low rate) and 50 gal (high rate) per acre and Methyl Bromide/Chloropicrin 67/33 at 175 lb (reduced rate) and 350 lb (full rate) per acre were tested. ‘Sunray’ yellow squash seeds were sown after the proper off-gassing period had passed with yield and vigor data taken and analyzed.

Both rates of the new DMDS fumigant produced plants with significantly better vigor ratings (rating=1.0) compared to the untreated control (rating=4.0) and the low rate of Methyl Bromide/Chloropicrin (rating=2.0). Both rates of DMDS had higher yields (>136.5 lb) than the untreated control (26.7 lb) and were similar to the high rate of Methyl Bromide/Chloropicrin (171.6 lb). The low rate of Methyl Bromide/Chloropicrin had a significantly lower yield (100.2 lb) than both the high rates of DMDS and Methyl Bromide/Chloropicrin. DMDS is not labeled and is currently for experimental use only.

Remember that fumigation is not the only strategy for limiting Phytophthora. Avoid rotating with other crops that are susceptible to Phytophthora. Irrigate conservatively only with non-surface water sources, and use drip irrigation whenever possible. Avoid moving farm equipment between infested and clean fields without proper cleaning, and never dump diseased plants and fruits onto a field. Apply fungicides early and often, and scout fields continuously for any sign of disease.

Remember that the pesticide label is the legal document on pesticide use. Read the label and follow all instructions closely. The use of a pesticide in a manner not consistent with the label can lead to the injury of crops, humans, animals, and the environment, and can also lead to civil or criminal fines or condemnation of the crop. Pesticides are good management tools for the control of pests on crops, but only when they are used in a safe, effective and prudent manner according to the label.

Effective fumigation requires:

  • Proper soil temperatures.
  • Proper soil moisture.
  • Good soil condition.
  • Moisture seal to keep fumigant in soil.
  • Proper application depth.
  • Proper off-gassing of product.

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