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.