Phytophthora Blight of Cucurbits
COOPERATIVE EXTENSION NEW YORK STATE CORNELL UNIVERSITY
Margaret Tuttle McGrath
Department of Plant Pathology, Long Island Horticultural Research and Extension Center
Phytophthora blight is a disease that should be a concern to all cucurbit growers. It has been described as the most destructive disease of cucurbits because nothing causes greater loss. Total crop loss has occurred in some fields (Fig. 1). While all cucurbits are susceptible, squash, cucumber and pumpkin seem to be affected more commonly than cantaloupe. It has been increasing in importance in the U.S. Fruit rot was first reported in Colorado and California in the late 1930's. The disease was found sporadically in most of the U.S. until the 1980's, except in California, where it occurred more regularly; then incidence increased notably in Florida, Georgia, North Carolina, New Jersey, Michigan, and the northeastern states. The increase followed a hurricane in some areas. Phytophthora blight also commonly affects pepper (click here for a description of the disease in pepper), and less commonly affects eggplant and tomato.
Symptoms and Signs
The pathogen, Phytophthora capsici, causes seedling damping-off, leaf spots, foliar blight, root and crown rot, stem lesions, and fruit rot. Leaf spots are dark brown and large (up to 5 cm in diameter)(Fig. 2). Crown rot causes the entire plant to completely collapse and die in a short period of time. Summer squash often die back from the growing tip (Fig. 3 and 4). Vines as well as growing tips can be affected in other cucurbit crop types. Affected vine tissue is brown, appears water-soaked, and often collapses (Fig. 5).
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Phytophthora fruit rot typically starts on the underside of the fruit that is in contact with the ground (Fig. 6). Initial symptoms can appear on the upper surface as a result of pathogen dispersal in splashing water. These can be a water-soaked spot (Fig. 7) that may have visible fungal growth (Fig. 8) or, less commonly, a depressed spot (Fig. 9). Lesions are soft and easily punctured when handled. Symptoms can also begin around the stem due to systemic infection from the vine (Fig. 10). Fruit can become completely affected and collapse (Fig. 11). Fruit symptoms also can develop rapidly after harvest. The fungus produces a white yeast-like growth that contains many sporangia, especially under moist conditions (Fig. 4, and Fig,8, and Fig11). Sporangia are lemon-shaped fungal structures that cause new infections when they are spread to healthy fruit, leaves, or vines. Sporangia either germinate and grow into these tissues, or several zoospores form inside each sporangium. Sporangia and zoospores are asexual spores, which means they are produced by an individual. Zoospores are released from sporangia in water and are able to swim for hours using their two flagella. They are capable of directional movement to a host based on chemical attraction.
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Two diseases that can be confused with Phytophthora fruit rot are Pythium fruit rot (Fig. 12), which is characterized by white fluffy growth resembling fine cotton (when new) or shaving cream (when old), and Sclerotinia white mold (Fig. 13), which is characterized by white dense cottony growth with black, hard, pea-like structures.
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Phytophthora capsici survives in soil between crops for more than two years. The pathogen can survive longer if thick-walled oospores are produced. Oospores are formed when mycelia of two individuals of opposite mating type (similar to male and female) grow together. Both mating types have been found in some fields, including fields in the northeastern U.S. Since oospores are the product of sexual reproduction (genetic recombination), they are the main source of new races or biotypes, including fungicide resistant biotypes.
Soil moisture conditions are important for disease initiation. Sporangia form when soil is at field capacity (within 24 hours under controlled conditions) and they release zoospores when soil is saturated (5 to 6 hours under controlled conditions). Further disease development in a crop can occur rapidly because sporangia are produced abundantly on affected fruit and they are dispersed under a wider range of conditions than zoospores. Ideal conditions for infection are wet soils above 65° F and air temperatures in the 75 to 85° F range.
Current recommendations center around preventing the pathogen from being moved into a new field and managing soil moisture to avoid saturated conditions which favor disease onset. It is important to use an integrated program with as many of the following practices as possible.
1. Select fields where Phytophthora blight has never occurred when possible. The fungus that affects cucurbits can also cause blight in pepper, fruit rot in eggplant, and buckeye rot in tomato. Buckeye rot can also be caused by Phytophthora parasitica. An effective rotational period has not been identified yet. Two years has been shown to be insufficient, therefore select a field where susceptible crops have not been grown for at least 3 years. Both mating types of the pathogen have been found in fields in New York, and also in other states. Therefore it is possible for the pathogen to produce oospores, which are capable of long-term survival. Rotation will be more successful where only one mating type occurs. Selected fields should be isolated from fields where Phytophthora blight has occurred to avoid the potential of the fungus being moved (in run-off or on farm equipment) from the infested field into the cucurbit crop. Late blight of potato and tomato is caused by a different fungus (P. infestans); therefore, previous occurrence of late blight is not a concern in field selection.
2. Select well-drained fields.
3. Make sure water will be able to drain out of the field. Use 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 at their ends, make a ditch or waterway across the end of the field for water coming out of the field in the trenches, and continually grade soil at the end to allow water to leave.
4. Physically separate plantings of susceptible crops (cucurbits, pepper, eggplant, and tomato). Plantings should be located such that there is no opportunity for water to move from one planting to another. The pathogen can also be dispersed in rain splash during storms. Therefore it is prudent to consider prevailing wind direction when deciding where to locate multiple plantings of susceptible crops on a farm.
5. When growing small-fruited pumpkins, select varieties producing hard, gourd-like rinds (such as Lil Ironsides). Mature fruit of such varieties are substantially less susceptible than varieties with conventional rinds which are softer (Fig. 14).
6. Minimize hardpans and plowpans by subsoiling or chisel plowing before planting.
7. Do not plant the crop in areas of the field that do not drain well (Fig. 15). Plant a cover crop in place of the crop in these areas.
8. Prepare raised dome-shaped beds for summer squash and other bush-type crops. Ideally beds should be a minimum of 9 inches high. Use a bed shaper to provide more lasting beds as opposed to a simple ridge. Use a transplanter that doesnt leave a depression around the base of the plant. Fill in any depressions. Raised beds are not recommended for vining crops as some fruit will develop in the low area between beds where conditions will be favorable for Phytophthora blight.
9. Minimize hardpans and plowpans by not driving through wet fields.
10. Clean farm equipment, shoes, etc. of soil between fields. Movement in soil on equipment and shoes probably is an important means by which Phytophthora has been spread between fields on farms and may account for the occurrence of Phytophthora blight in fields with no previous history of susceptible crops.
11. Subsoil between rows after planting and before vining to improve drainage (Fig. 16). Subsoil again as needed after rain. Good drainage is also important for driveways in fields, as symptoms have been observed first on plants next to the compacted soil of driveways (Fig. 17), therefore, subsoiling along the edge of driveways is also needed. It is preferable to plan driveways before seeding leaving ample space, instead of seeding the entire field and then driving over plants.
12. Avoid over irrigating. Normal irrigation practices usually do not encourage Phytophthora blight except when leaks leaks and puddles occur (Fig. 18). Do not irrigate at night time when temperatures are above 70° F.
13. Do not irrigate from a pond that could contain water that drained from an infested field.(Fig. 19)
14. Fungicides have provided minimal control in efficacy trials and therefore should not be used alone without the cultural practices listed here. None of the fungicide programs evaluated in New York and elsewhere in the U.S. provided the level of control considered necessary for commercial production. Fungicides that have been tested include Bravo, Aliette, Ridomil, Acrobat, copper fungicides, and new experimental materials. Aliette is registered for managing this disease. Bravo and copper fungicides can also provide some control; they are registered for other diseases. A preventive spray program is expected to be more effective than waiting until symptoms occur.
15. Scout fields for symptoms routinely, especially after major rain storms. Include any areas where water did not drain well and near the end of irrigation pipe.
16. When symptoms are localized in a small area of a field, disking the area is worthwhile (Fig. 20). Begin with a border of healthy-appearing crop around the affected area.
17. Do not discard cull fruit in the field, including fruit that are healthy but over-sized or over-ripe (Fig. 21).
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18. Fruit that look healthy should be removed from infested fields as soon as possible and checked routinely for symptom development so that fruit developing symptoms after harvest can be discarded before the fungus spreads further. Asymptomatic affected fruit should develop symptoms within a week. It is especially important to harvest before rain. Growers have asked about disinfectants. None are registered for this use. Furthermore, applying a disinfectant to fruit will only kill Phytophthora spores on the fruit at the time; it will not stop the fungus if it has already started to infect the fruit and it will not affect spores that land on the fruit after treatment.
19. Do not display pumpkin fruit for sale in a field where Phytophthora blight developed in previous years: healthy fruit have developed fruit rot in these situations.
20. Do not save seed from a field where Phytophthora blight occurred.
Other practices that will improve water management will help control Phytophthora blight.
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