Have you ever noticed a low spot in a pumpkin field that just ‘goes
down,’ all of a sudden, or an area where pepper plants become wilted and
stunted, maybe on some poorly drained soil?
If so, pay special attention because those are signs of Phytophthora
blight. The soil-borne fungus causing blight, which goes by the scientific
name Phytophthora capsici, can inflict extensive crop loss, especially
if it gets established and spreads before the farmer realizes what’s going
on. In cucurbits, symptoms of the disease include crown rot, tip blight,
leaf spots, and fruit rot. The same disease organism also leads to blight
in pepper, fruit rot in eggplant, buckeye rot in tomato, and pod blight
in lima bean. However, it is not the same Phytophthora that causes late
blight in potato.
Dr. Meg McGrath of Cornell’s Long Island Horticultural Research and
Extension Center has been studying this disease and how to manage it. I’ve
summarized her recommendations, which are primarily preventative
- and that requires planning ahead.
Use multiple strategies. Since no single management practice will
effectively control Phytophthora blight, it’s essential to use an integrated
approach with many of the practices listed below.
Why is prevention key? Once Phytophthora blight occurs on a farm
it is challenging to continue growing susceptible crops without reoccurrence
of the disease. Prevention is also important because the disease is difficult
to suppress with fungicides once it starts to develop in a field.
Plan rotations carefully. If possible, plant susceptible crops in
fields where Phytophthora blight has never occurred. It isn’t clear just
how long a rotation period is needed, but two years have been shown to
be insufficient. Plan on a minimum rotation of 3 years without planting
any host crops in a given field.
Plantings of the different susceptible crops (cucurbits, pepper,
eggplant, lima bean, and tomato) should be separated enough so that there
is no opportunity for water to move from one planting to another. Since
the pathogen spreads in runoff and rain splash during storms keep in mind
the slope of the land and the prevailing wind direction when deciding where
to locate multiple plantings of susceptible crops on a farm.
Avoid standing water. This disease thrives in wet soil, so it is
critical to avoid standing water in your fields. If possible, plant susceptible
crops only on well-drained soils. Disrupt hardpans and plowpans that interfere
with drainage by subsoiling or chisel plowing before planting. And don’t
make compaction worse by driving on wet soil.
Take steps to assure that excess water will be able to drain out
of the field. This may include using a land plane to level the field as
much as possible. If water does not normally drain out of the field, then
make a trench between beds or rows and make a ditch or waterway across
the end of the field to capture water coming out of the trenches.
If necessary, subsoil during the growing season. Go between rows
after planting but before vines run to improve drainage. Subsoiling along
edges of driveways may also be a good idea, since disease symptoms have
been observed to start on plants next to compacted driveways. Plan the
location of driveways in the field before you seed vining cucurbits, leaving
ample space for equipment, instead of seeding the entire field and then
driving over plants.
If there are areas in a field that have not drained well in the past,
and probably never will, don’t plant them at all. Instead, plant a cover
crop rather than a susceptible crop.
Your local NRCS office (Natural Resources Conservation Service) can
help you decide on the best practices for improving drainage on your farm,
and you may be able to apply for cost-sharing to help you implement these
Irrigate carefully. While normal irrigation practices usually do
not encourage Phytophthora blight, be aware that over-irrigation, or steady
leaks in the irrigation system can provide enough excess soil moisture
for the disease to get established. Be sure not to irrigate from a pond
that contains waters that drained from an infested field!
Hard rinds make a difference. When growing small-fruited pumpkins,
select varieties producing hard, gourd-like rinds (such as Lil’ Ironsides).
These have been shown to be substantially less susceptible than varieties
with conventional rinds.
Plant smart. Prepare raised, dome-shaped beds for non-vining crops.
Ideally, raised beds should be at least nine inches high. Use a bed shaper
to provide more lasting beds as opposed to a simple ridge. Use a transplanter
that doesn’t leave a depression around the base of the plant.
If you have it, don’t spread it. Phytophthora can easily move between
fields on a farm. To contain the disease it is critical to clean equipment
and boots after working in an infested field. Movement in soil on equipment
and shoes is probably an important means by which Phytophthora has been
spread on farms and may account for its occurrence in fields with no previous
history of susceptible crops.
Don’t rely on sprays. Fungicides have provided minimal control in
efficacy trials and therefore should not be used alone without the cultural
practices listed here. As with most diseases, a preventive spray program
can be expected to be more effective than waiting until symptoms occur
and trying to treat them.
Keep an eye out for trouble. Be sure that someone on your farm
is familiar with the disease and scout fields for symptoms on a routine
basis. It is especially important to scout shortly after heavy rains, paying
careful attention to areas where water did not drain well. Contact the
plant diagnostic lab in your state if you need help identifying a disease.
Practice good sanitation. When symptoms are localized in a small
section of a field, disk the area in to ‘clean it up.’ Begin with a border
of healthy-appearing crop around the affected area.
Do not put discarded cull fruit in the field, whether it’s diseased
or healthy but over-sized or over-ripe. Fruit that look healthy should
be harvested from infested fields as soon as possible. It is especially
important to harvest before rain. After harvest, check fruit so those with
developing symptoms can be discarded before the fungus spreads further.
Any fruit that looks OK at harvest but was infected should develop symptoms
within a week.
When displaying pumpkins for sale, don’t place them in contact with
the ground in an area where Phytophthora blight developed in previous years.
And, do not save seed from a field where Phytophthora occurred.
For more information, including a photo gallery of Phytophthora blight
symptoms and management, see Cornell’s ‘Vegetable MD’ web site at: http://vegetablemdonline.ppath.cornell.edu