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Phytophthora Blight of Cucurbits and Pepper
Phytophthora blight, caused by the fungus-like pathogen
Phytophthora capsici, was detected on pepper, pumpkin,
squash, gourds and eggplant in British Columbia, for the first
time in 2004. It was confirmed in two neighbouring market
gardens in the Kelowna area, where it caused significant damage.
Phytophthora blight is a serious threat to production of
susceptible crops worldwide, particularly cucurbits and
solanaceous plants. It is a fast spreading, aggressive
disease, capable of causing complete crop failures. The
disease has been increasing in severity in the United States in
recent years, where outbreaks have threatened the survival of
the processing pumpkin industry. Many vegetable growers are
familiar with a close relative of this disease – late blight of
potato and tomato, caused by Phytophthora infestans.
Hosts
Crops that can be infected by Phytophthora capsici blight
include pumpkin, many types of squash, gourd,
watermelon, cantaloupe, honeydew melon, cucumber, peppers, eggplant and
tomato. In 2004, US researchers reported that beet, Swiss chard, lima
beans, turnip and spinach were also susceptible. In total, there are
over 50 susceptible species, including many common
weeds.
Symptoms
![](/web/20061229051059im_/http://www.agf.gov.bc.ca/cropprot/images/pumpkin1.jpg) |
![](/web/20061229051059im_/http://www.agf.gov.bc.ca/cropprot/images/pumpkin2.jpg) |
Infected pumpkin fruit covered with white
cottony growth and sporangia of Phytophthora capsici. |
Infected pumpkin fruit
turned completely white by growth of Phytophthora capsici |
Phytophthora capsici may affect all parts of the plant, causing a wide variety of
symptoms. It may cause pre- and post-emergence damping-off, stem
and vine blight,
wilting or fruit rot. Symptoms can appear as fast as 3 to 4 days after
initial infection when temperatures are warm.
Damping-off may occur both before and after emergence of
seedlings in susceptible crops in the spring. Symptoms include a watery rot near the soil line,
wilting, and subsequent plant death. White fungal growth may
appear on infected
areas of blighted seedlings under moist conditions. Damping-off
is more likely to occur when soil conditions are wet and warm
(20 to 30°C), and when the disease is well established in the
soil. Many other fungi and fungus-like organisms can also cause damping-off, including
Pythium, Rhizoctonia and Fusarium species. Damping-off caused by
P. capsici has not yet been found in British Columbia. It is
possible that local spring soil temperatures may not be warm enough to favour
early infection.
Cucurbits
All cucurbits are susceptible to Phytophthora rot, but squash
and pumpkin are the most commonly affected. Cucumber and melon are
considered to be somewhat tolerant.
Foliar symptoms on leaves and petioles appear as rapidly
expanding, irregular, water-soaked lesions, resulting in a rapid
collapse and death of leaves. Leaf spots are chlorotic (yellow)
at first and then turn brown with yellow or light green borders.
Vine blight appears as water-soaked lesions on the vines.
Lesions turn brown and necrotic within a few days, resulting in
stem girdling, wilting and death of foliage above the lesions.
Dieback of shoot tips, wilting, shoot rot, and plant death
quickly follow initial infection. P. capsici can
devastate entire squash plantings in a matter of days when
conditions are warm and moist.
Fruit rot was the predominant symptom seen on pumpkin,
squash and gourds during the Kelowna
outbreak in 2004. Fruit rot often
starts on the underside of the fruit where it sits on the soil.
It can also develop on the upper side of the fruit following
rain or overhead irrigation. Early symptoms include large,
water-soaked or slightly sunken, circular lesions, which expand
to cover the fruit with white mold. The mold consists of
millions of sporangia (spores), which can spread with wind and
rain to cause further infections. The white fungal growth of
P. capsici on the fruit should
not be confused with the white growth of powdery mildew, which
is a common problem on cucurbit leaves. Fruit rot progresses rapidly,
resulting in complete collapse of the fruit and invasion of
secondary rots. Fruit rot can also develop after harvest.
![](/web/20061229051059im_/http://www.agf.gov.bc.ca/cropprot/images/squash.jpg) |
![](/web/20061229051059im_/http://www.agf.gov.bc.ca/cropprot/images/gourd.jpg) |
Yellow scallop squash fruit covered with
white cottony growth and sporangia of
Phytophthora capsici |
Gourd fruit infected with
Phytophthora capsici |
Pepper
On pepper, infection of the stem near the soil line is common.
Stem lesions start as dark, water-soaked areas which become
brown to black and result in girdling, wilting and plant death. P. capsici
may also cause root rot and foliar blight on pepper. On
leaves, small, water soaked lesions expand and turn a light tan
colour. White moldy growth may be seen on leaves during wet
periods. Rapid blighting of leaves and shoots may occur. Pepper
fruit can also be infected through the fruit stalk. Fruit rot
appears as dark green, water-soaked areas that become covered
with a white to gray mold. Infected fruit dries, becomes
shrunken and wrinkled, and remains attached to the stem.
![](/web/20061229051059im_/http://www.agf.gov.bc.ca/cropprot/images/pepper3.jpg) |
![](/web/20061229051059im_/http://www.agf.gov.bc.ca/cropprot/images/pepper1.jpg) |
Pepper plants killed by Phytophthora
blight |
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Eggplant
Fruit rot is the most common symptom of phytophthora blight in
eggplant. Symptoms appear as a round, dark brown area on the
fruit, which is surrounded by a rapidly expanding lighter tan
zone. Fruit lesions and eventually whole fruit may be covered
with white to gray moldy growth during wet periods.
![](/web/20061229051059im_/http://www.agf.gov.bc.ca/cropprot/images/eggplant2.jpg) |
![](/web/20061229051059im_/http://www.agf.gov.bc.ca/cropprot/images/eggplant.jpg) |
Eggplant fruit showing symptoms of
Phytophthora blight infection in the field. |
Infected eggplant fruit showing
discolouration and light sporulation of Phytophthora capsici. |
Tomato
Infection of field tomatoes was not observed in BC in 2004,
although tomato crops were grown near infected peppers, pumpkins
and squash. However P. capsici does cause serious problems in tomatoes in other areas.
Phytophthora blight can cause crown rot, leaf spot, foliar
blight and fruit rot in tomatoes. Fruit rot begins as dark,
water-soaked spots, often where fruit is touching the soil. The
infected spot rapidly expands during warm weather to cover most of the
fruit surface with a brown, watery discoloration that may appear
as
concentric rings. Under humid conditions, infected fruit may be
covered with white moldy growth and rot entirely following
invasion by secondary microorganisms. Similar symptoms can also
be caused by the late blight pathogen, Phytophthora infestans.
Life Cycle
P. capsici is a soilborne pathogen which
overwinters as oospores (thick-walled resting spores) in the
soil or in plant debris. Oospores are resistant to desiccation
and
cold temperatures, and can
survive in the soil for many years.
In the spring, oospores germinate to produce sporangia and
zoospores (asexual spores) when soil moisture is at field
capacity. Sporangia are spread by wind and water through the
air and are carried with water movement in soil. Sporangia
germinate to directly infect host tissue, or if conditions are
wet, they can also germinate to release zoospores. Zoospores are
motile and swim to invade host tissue. P. capsici can
also be spread in infected transplants, seed, and through
contaminated soil and equipment.
Abundant sporangia are produced on infected tissues,
particularly on infected fruit. Sporangia are spread in water,
by rainsplash, or in air currents. Wind-borne sporangia can be
carried long distances. If the environmental conditions are
favourable, the disease develops very rapidly.
Phytophthora blight is favoured by high soil moisture,
frequent rains or irrigation, and warm temperatures (optimum
24-33 oC). The disease is usually associated with heavy rainfall,
excessive-irrigation, or poorly drained soil. P. capsici
does not survive cold temperatures very well unless oospores are
present.
Pathogen variation and strains
P. capsici shows considerable genetic variation. Different
pathogenic strains may have the ability to infect different
crops, and there are also differences in virulence, or the
ability to cause disease in host plants. Some strains may be
more aggressive than others on certain hosts.
Limited pathogenicity tests were conducted at the Pacific
Agri-Food Research Centre using P. capsici isolates collected from
pumpkin and squash in Kelowna in 2004. The BC isolates were able to
cause infection of sweet pepper, winter squash and golden
zucchini, but did not infect musk melon.
P. capsici has 2 mating types, A1 and A2. When both
mating types are present in the same field, the pathogen is able
to reproduce sexually and produce oospores – a type of spore
that can survive for many years in the soil. To date, only one mating type has been detected in B.C. from
the 2004 outbreak.
Prevention
P. capsici had never been reported in British Columbia
before 2004, and the 2004 outbreak was very small and localized.
The disease was not detected in 2005. Some precautions can be
taken to avoid introducing it to your farm.
Seed Source: The disease may have been introduced to the
Kelowna area on infected seed. Use a reliable source for
disease-free seed and transplants. Do not collect seed from an
infected field.
Scouting: Early detection may help to avert serious losses.
Scout your field regularly for disease symptoms. Pay particular
attention to low areas of the field where the soil remains wet
for longer periods of time.
Identification: Submit suspected P. capsici infected plants
to the BCMAL Plant Diagnostic Laboratory or contact a BCMAL
Plant Pathologist for disease diagnosis. Proper identification
of pests and diseases is an important component of
integrated
pest management.
Biosecurity: Take precautions to prevent spreading diseases
between fields, and to prevent possible introductions of
diseases from fields of other growers. Be aware that
Phytophthora may be carried on clothing, foot-ware and farm
equipment. Refer to the publication: Biosecurity Guidelines for more
information.
Disease Management
Phytophthora blight is a difficult disease to control,
particularly once established in the soil as oospores.
Management strategies should combine cultural and chemical
controls, along with other disease prevention measures.
- Crop rotation is an excellent disease management strategy
for most vegetable diseases. Rotate to non-susceptible or
non-host crops for
at least 2 years. Be sure there is no crop residue left from
previous infected crops before replanting. Note, crop rotation
is not effective in areas where oospores are present in the
soil. When soil is infested, it may be best to move production
of susceptible crops to a field with no history of the disease.
Currently it is not known whether the disease has successfully
overwintered in Okanagan soils.
- Control volunteer crop plants and susceptible weeds such as
nightshade during crop rotations. Control weeds during the
growing season.
- Plant resistant varieties, if available. Some pepper
varieties have tolerance to Phytophthora blight. Check seed
suppliers for resistance ratings. There are no cucurbit
cultivars with measurable resistance currently available.
- Select well-drained fields, and avoid planting into
low-lying areas. Raised beds are recommended for non-vining
cucurbits.
- Do not over-irrigate. Discontinue overhead irrigation if
the disease is present.
- When symptoms are localized in a small area of the field,
disk the area. This will help to prevent movement of spores from
infected plants to healthy plants during subsequent rainfalls.
- Clean equipment before moving it from infested to clean
areas.
- Do not work in wet fields.
- Do not keep cull piles. Bury or remove infected plant
material from the vicinity of fields and vegetable
stands/display areas.
- Remove healthy fruit from the infested area as soon as
possible and check them periodically for symptoms. Cull all
fruit with symptoms, and do not leave culls on the field.
- There are no fungicides registered for control of P. capsici
blight in Canada, and fungicides have not been highly
effective in other areas. However fungicides applied for other
diseases may provide some level of control, particularly
fungicides that are effective against late blight or downy
mildew. Preventive sprays are more effective than spray
programs started after the disease symptoms are already present.
For best results, the use of fungicides should always be
combined with other disease management practices. Consult the
Vegetable Production Guide for current fungicide
recommendations.
Links for Further Information
-
Phytophthora Blight: A Serious Threat to Cucurbit Industries,
by Mohammad Babadoost, University of Illinois – APSnet
-
Scary Diseases Haunt Pumpkins and Other Cucurbits – APSnet
-
Phytophthora Blight of Cucurbits, Pepper, Tomato, and
Eggplant - Thomas A. Zitter, Department of Plant Pathology,
Cornell University
- Vegetable Diseases Caused by Phytophthora capsici in Florida,
by P.D. Roberts, R.J. McGovern, T.A. Kucharek, and D.J. Mitchell
– University of Florida
-
Greenhouse and Field Evaluation of Bell Peppers for
Resistance to Phytophthora Blight, by
M. Babadoost, S. Z. Islam, and M. Hurt - University of Illinois
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