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Disease Names

Fruit rot of peppers (Capsicum species)

Leaf spot and blight of peppers

Collar rots and stem rots of pepper

Green wilt of pepper

Damping-off and root rots of peppers

Hosts

Phytophthora capsici is known to infect many species of pepper, tomato, and other members of the solanaceae family. Melon, cucumbers, and other members of the plant family cucurbitaceae are also attacked.

Distribution

Phytophthora capsici is frequently isolated from pepper and tomato on Oahu and is likely to be present on other islands as well. It causes severe losses of pepper in the southwestern U. S. and fruit rots (pepper, tomato, eggplant) in Florida. The fungus is reported from the Western hemisphere, Asia, and Europe.

Symptoms

Epidemics of pepper fields are caused by Phytophthora capsici during disease conducive wet weather. Leaf blights begin as small water-soaked areas on the underside of leaves. These enlarge to form irregular spots of various shapes and sizes. Infected leaf tissue is wilted, light green or gray-green, later becoming tan to white and scalded in appearance. With moisture, leaf spots have a water soaked border. 

The pathogen moves into stems from infected leaves and sections of the plant are killed. Rots which develop at the soil line or that affect major branches cause the plant to wilt or die. Root rots can be severe and stunt the plant or cause plant decline. Fruit rots are irregular in shape and olive green or light green with water soaked borders. Rots expand rapidly and fruits can be completely diseased and desiccated, causing the formation of "mummified" fruits. Infected seeds are brown and shriveled.

Biology

Taxonomy

For many years, controversy has surrounded the inclusion of Phytophthora palmivoramorphological form IV (MF 4) strains into Phytophthora capsici. While the MF 4 strains share some morphological characteristics, with Phytophthora capsici, they remain distinguishable but with overlapping host ranges. Refer to Phytophthoratropicalis for more information.

Biology

Phytophthora capsici produces microscopic, asexual spores called sporangia. Sporangia are nearly spherical to pyriform (pear shape), hyaline (colorless), papillate(pointed at the tip), deciduous (spores fall from the colony) and have a long pedicel (stalk) attached to the base of the spore. In water, sporangia form and release several swimming spores called zoospores. Sporangia also germinate directly by producing several germ tubes that begin new fungal colonies. 

Phytophthora capsici is heterothallic and has two mating types (A1 and A2). Both mating types are needed for the abundant production of sexual spores called oospores. Oospores are thick walled and form within a mother cell called the oogonium, which is red or orange in some types of media. Some species of Phytophthora are induced to form oospores when the mating type of another species is present.

This pathogen grows well between 25° and 30° C.

Epidemiology

In pepper fields, the fungus is soil borne and initial infections of roots, collars, and lower leaves occur. The fungus grows within the host and produces sporangia on the surface of diseased tissue, especially leaves. Sporangia are spread by splashing water from irrigation or rain. With moisture present, zoospores are formed and released. These zoospores swim for a few minutes to more than an hour before encysting. Environmental conditions such as water temperature, nutrition, pH, and other factors determine the length of time zoospores continue to swim. Encysted zoospores germinate by producing a thin fungal hypha or thread. The germ tube commonly penetrates the leaf through stomates which are natural opening in the leaf epidermis. Larger germ tubes produced by sporangia also penetrate the leaves.

Sporangia are also moved within the field by contact with field equipment, clothing, gloves, tools, etc. Movement of soil from one field to another on equipment or boots will move this pathogen. 

The fungus survives in the soil in host debris. Roots, stems, and mummified fruits left in the field after harvest, harbor the pathogen for months. Phytophthora capsici is also seed borne.

Management

Rotation with non-susceptible crops will reduce the amount of Phytophthora capsicisurviving in soil. Fresh, clean seeds should be planted in new potting mix to establish healthy transplants. Monitor seedlings as well as the field and remove diseased plants as soon as they occur. Some cultivars are more tolerant, and better resistance in pepper to Phytophthora capsici is being developed.

References

Farr, D. F., G. F. Bills, G. P. Chamuris and A. Y. Rossman. 1989. Fungi on Plants and Plant Products in the United States. APS Press. St. Paul, Minnesota. 1252 pp.

Uchida, J. Y. and M. Aragaki. 1980. Chemical Stimulation of oospore formation in Phytophthora capsici. Mycologia 72:1103-1108.

Walker, J. C. 1952. Diseases of vegetable crops. McGraw-Hill Book Co. Inc. The Maple Press Co. York, PA.

COPYRIGHT: Janice Y. Uchida

FILE: BP-CAPS

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