SPOTS on leaves, shoots, and potato tubers. Spots on the upper leaf side at first small, yellowish, then gradually enlarging and browning; at favourable conditions, the spots quickly cover the whole leaf area, and the plants die. On the under leaf surface, especially at the border of the affected and healthy leaf tissue, a delicate, white coat composed of sporangiophores with sporangia is formed.

SPORANGIOPHORES (sp) WITH SPORANGIA (s) up to 1000 µm long, ca. 10 µm wide at the base, sympodially branched 1-5 times. The main axis of the sporangiophores and its branches are of indeterminate growth. After the formation of sporangia at the tips of the sporangiophore branches, apical growth of the sporangiophore continues. The sporangia fall off from a lateral position, and then the sporangiophore produce a sympodium with characteristic swellings at the nodes.

SPORANGIA (s) egg-shaped or lemon-shaped, occasionally elliptical or subglobose, 22-32 x 16-24 µm, with a small stalk at the base and a papilla at the top.

OOSPORES globose, 24-35 µm diam, of a thick, two-layered wall. The oospores germinate to form sporangia with zoospores.

PLANT HOST AND DISTRIBUTION. The plant hosts of P. infestans are different species of the family Solanaceae. In Poland, the fungus has been found to affect, e. g., Datura metel L., Lycium halimifolium Mill., Lycopersicon esculentum Mill., Nicotiana affinis hort., Petunia hybrida hort., Solanum aviculare Forst., S. dulcamara L., S. melongena L., S. nigrum L. em. Mill., and S. tuberosum L. (Kochman and Majewski 1970).

The fungus has a worldwide distribution (Agrios 1988; Brooks 1953; Kochman and Majewski 1970; Smith et al. 1988).

NOTES.The genus Phytophthora is distinguished from the true downy mildews by its indeterminate branched sporangiophores and from the genus Pythium by the complete differentiation of motile zoospores within sporangia before expulsion (Smith et al. 1988).

Phytophthora infestans is an atypical member of the genus, being one of only two species that can shed sporangia easily into the air.

Phytophthora infestans is the notorious cause of late blight of potato. In 1845 and 1846, the disease resulted in the deaths of ca. 1 million Irish people and the emigration of up to 1.5 million individuals, mainly to North America.

Phytophthora infestans has originally been described as Botrytis infestans in 1845 by C. Montagne, a retired French army doctor.

Phytophthora infestans may kill the foliage and stems of potato and tomato plants at any time during the growing season. The fungus also affects potato tubers and tomato fruits in the field, which rot in the field or in storage.

In the field, affected plants die within a week or two when weather conditions are favourable.

From G. L. Schumann

In case of late blight of potato, P. infestans overwinters in the form of mycelium in diseased tubers left in the field. Then, the mycelium systemically invades the shoot of the tuber growing in the spring and the fungus produces sporangiophores on the aerial parts of plants. The sporangiophores emerge through stomata of the shoots and the under leaf side and then produce sporangia. The optimal conditions for production of sporangia are a high relative humidity (with an optimum of 100%) and temperatures of 12-16oC. Infection of neighbouring plants takes place by means of sporangia transported by water or wind. At temperatures up to 12-18oC, sporangia germinate almost entirely to produce zoospores, while above 15oC sporangia germinate directly by forming a germ tube, and, hence, they function like a conidium. Each sporangium usually produces 3-8 zoospores. The fungus penetrates into its plant host through the leaf cuticule or a stoma. Then, it produces mycelium that grows between the cells and finally sends long, curled haustoria into the cells. When the infected cells begin to decay, the mycelium spreads peripherally into fresh tissue. A few days later, new sporangiophores are produced. In favourable weather, the period from infection to sporangia formation may be as short as four days.

The second phase of the disease is the infection of tubers. During wet weather, sporangia are washed down from the leaves into the soil. The tubers formed near the surface of the soil are infected by zoospores. The sites of penetration are lenticels or wounds.

REFERENCES

Agrios G. N. 1988. Plant pathology, 3rd edition, Academic Press, INC. San Diego, New York, Berkeley, Boston, London, Sydney, Tokyo, Toronto.

Brooks F. T. 1953. Plant diseases. Geoffrey Cumberlege. Oxford University Press. London, New York, Toronto.

Kochman J., Majewski T. 1970. Grzyby (Mycota) IV. Glonowce (Phycomycetes), Wroslikowce (Peronosporales). Warszawa, 308 pp.

Schumann G. L. 1990. Selected plant pathogenic lower fungi. APS Press Slide Collections.