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|Late blight of
Late blight of tomato
TYPE: Kingdom: Stramenopila
Phylum: Oomycota; Order, Peronosporales; Family, Pythiaceae.
Late blight of potato
Late blight of tomato
Primary hosts are potato (Solanum tuberosum) and tomato (Lycopersicon esculentum).
Worldwide. The recent distribution of the A2 mating type has had significant impacts on disease severity and incidence.
Phytophthora infestans causes serious losses of potato crops worldwide and is probably the most important pathogen of potato and tomato today. The disease, late blight, is famous for the destruction of the potato crops in Ireland in the 1840s and the resulting famine and death of over a million people. Today, epidemics recur in disease conducive environments.
The development of leaf rots or lesions varies with environmental conditions. The relative humidity, temperature, light intensity, and potato cultivar all affect symptomatology. Leaf lesions begin as small, irregularly shaped, light green to gray spots. In cool (20 C or less), moist (95-100% RH) environments, lesions expand rapidly to form large black rots (blights) that spread throughout the leaf and into petioles (leaf stem) and into the stem of the plant. Apical tips, flowers, and young stems are very susceptible (Fig. 1). Large sections of the plant are rotted and plants are eventually killed.
Rots of the tuber also develop and are characterized by slightly sunken areas that have a brown to purplish skin. On young tubers, infections are dark, reddish brown and 5 to 15 mm deep. In stored tubers, rots are sunken, dry, and light brown but infected areas are not distinct from healthy tissue. Secondary fungal and bacterial invasion frequently occurs and the entire tuber becomes rotted.
Phytophthora infestans produces microscopic, asexual spores called sporangia. These sporangia are hyaline (clear), lemon-shaped and 20-40 um long. When placed in water or in very high relative humidity, the cytoplasm in the sporangia divide and many swimming zoospores emerge from each sporangium. Sporangia are formed on specialized branches called sporangiophores. The branched sporangiophore, with swellings at the points where sporangia were attached are distinctive for Phytophthora infestans and useful for identification of this pathogen. In the absence of sufficient water or with temperatures above 24 C, no zoospores form. However, sporangia germinate by producing germ tubes that penetrate the host.
Both A1 and A2 compatibility types need to be present for sexual spores or oospores to be produced. Both compatibility types must infect the same plant or tuber for oospores to form in the field. A single oospore is produced within a larger oogonium (mother cell) and the antheridium (male cell) is at the base. Oospores have thickened walls and are believed to survive in soil between cropping seasons.
Field collected sporangia from tomato late blight are shown in Fig. 2, and a mature swollen sporangiophore is shown in Fig. 3.
In tropical environments, potato crops are grown continuously, thus aerial inoculum generally initiates the first incidence of disease in a new field. In temperate areas, soil or plant debris are believed to harbor the pathogen between seasons. The fungus will also survive in infected tubers that remain in soil from the previous season. Seed pieces can also be infected and harbor the pathogen. When new shoots are produced from infected seed pieces or older tubers, the fungus infects then sporulates on this new growth and the sporangia are then spread in the air or by water.
Cull piles left from the previous season will also produce initial inoculum for new crops. Tremendous amounts of spores can develop on diseased sprouts emerging from cull piles. During the growing season, cull piles continue to produce sporangia as the fungus invades and sporulates on any new shoots that develop.
When the environment is highly conducive for disease, sporangia are airborne and spread for miles. Sporangia land on healthy foliage and germinate in the presence of moisture. In low temperatures, if adequate water is available, zoospores are formed and released. These zoospores swim in water and can migrate over the wet leaf surface. Coming in contact with the host tissue, they encyst, and germinate. The germ tube formed by zoospores penetrate the leaf epidermis at multiple sites causing small spots. These small rots expand rapidly and leaf spots and blights (large rots) develop. At close to 100% relative humidity, Phytophthora produces abundant amounts of sporangia on the surface of the leaves. Sporangial masses are white on dark colored leaf lesions. These are blown to healthy plants and continue the disease cycle.
In the tropics, chemical spray programs must be implemented before disease is observed, especially during cool winter periods. Once foliar infection develops epidemics can become uncontrollable. Keeping the number of leaf rots low is crucial.
Good coverage with fungicides is important. Systemic fungicides such as metalaxyl move upwards in the plant, thus if only the top leaves are sprayed, the lower canopy will be unprotected and are ideal for disease development. Good coverage is also important if systemic fungicides are rotated with contact fungicides, such as mancozeb.
Avoid introducing new genotypes of Phytophthora infestans by planting only disease free seed pieces. Movement of the A2 mating type to Hawaii will likely occur on seed pieces. The present population of Phytophthora infestans is A1. Resistance to metalaxyl has been associated with the A2 mating type.
Prevent tuber infection by maintaining soil on plant hills. Keep tubers covered and kill vines 2 or more weeks before harvest to reduce the amount of Phytophthora infestans in the field at harvest. Remove infected tubers during storage.
Use cultivars that are more resistant in the tropics. Two types of resistance are recognized in potato: 1) specific resistance (also called vertical, race specific, oligogenic, or monogenic resistance, and 2) general resistance (also called horizontal, race nonspecific, or polygenic resistance). Specific resistance was discovered in the genes from Solanum demissum and potato breeders incorporated those resistance genes in new cultivars. Unfortunately, Phytophthora infestans is highly variable and all new cultivars with specific resistance eventually succumb to this pathogen. Some cultivars with moderate levels of general resistance are available and should be planted where possible. Since cultivar characteristics are developed in temperate environments, growers in the tropics must test several cultivars to determine which will provide the best yield, good growth, insect resistance, as well as tolerance to Phytophthora.
1. Agrios, G. N. 1988. Plant Pathology. APS Press. St. Paul, Minnesota.
2. Alexopoulos, C. J., C. W. Mims, and M. Blackwell. 1996. Introductory Mycology. John Wiley & Sons, Inc. New York, USA.
3. Hooker, W. J. 1986. Editor. Compendium of Potato Diseases. American Phytopathological Society Press. St. Paul, Minnesota.
COPYRIGHT: Janice Y. Uchida
Department of Plant Pathology
University of Hawaii