Oogonia:
Oogonia are rare in a host and in single cultures. Oogonia are 31–50 μm in diameter (average 38 μm). Oogonia develop promptly when A1 and A2 mating type isolates are paired.

Oospores:
Oospores are 24–56 μm in diameter (average 30 μm) (Fig. 4). Oospores are aplerotic.

Host Range and Distribution

Symptoms

Lesions are formed on leaves, stems, and tubers (Figs. 5–7). The leaf foliage can show symptoms at any time during the infection. Infections usually occur at the tip of the leaf margin and leaves develop purple-black or brownish black lesions. Infected foliage first becomes yellow and then water-soaked and eventually turns black. Masses of sporangia appear on the underside of the leaf. Tubers are infected later in the season as sporangia and zoospores wash from the leaf surfaces. Purple-brown blotches appear and then tubers decay. A wet rot occurs and can spread in storage. On tomato, leaf lesions are grayish brown and petioles and stems may become infected (Fig. 8). Brown zonate lesions form on tomato fruit (Fig. 9).

Sporangia can be spread by wind and rain. The pathogen overwinters in the soil in infected tubers or as oospores where both mating types occur. Movement of infected tubers has been a major source of dissemination of the disease. Volunteer plants, cull piles, and neighboring fields can be sources of inoculum.

References

Berkeley, M. J. 1846. Observations, botanical and physiological on the potato murain. J. Hortic. Soc. London 1:9-34.

Blackwell, E. 1949. Terminology in Phytophthora. Mycol. Pap. 30. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.

Caspary, R. 1852. Page 327 in: Verhandlungen des Verein zur Beförderung des Gartenbaues in den Koniglich Preussischen Staaten (Proceedings of the Association for Horticulture in the Royal Prussian States). (In German)

Cline, E. T., Farr, D. F., and Rossman, A. Y. 2008. A synopsis of Phytophthora with accurate scientific names, host range, and geographic distribution. Plant Health Progress doi:10.1094/PHP-2008-0318-01-RS.

de Bary, A. 1876. Researches into the nature of the potato fungus, Phytophthora infestans. J. R. Agric. Soc. Engl., Ser. 2 12:239-269.

Montagne, J. F. C. 1846. Note sur la maladie qui ravage les pommes de terre et caractčres du Botrytis infestans (Note on the disease that ravages potatoes and is characterized as Botrytis infestans). Mem. Inst. Fr. 609:98-101, 312-313. (In French)

Morren, C. 1844. Notice sur le Botrytis devastateur ou le champignon des pommes de terre. Ann. Soc. R. Agric. Botanique Gand. 1:287-292.

Stamps, D. J. 1984. Phytophthora infestans. CMI Descr. Pathog. Fungi Bact. 838:1-2.

Stamps, D. J., Newhook, F. J., Waterhouse, G. M., and Hall, G. S. 1990. Revised tabular key to the species of Phytophthora de Bary. Mycol. Pap. 162. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.

Unger, F. 1847. Beitrag zur Kenntnis in der Kartoffelkrankheit vorkommenden Pilze und der Ursache ihres Entstehens (Contributions to the knowledge of the fungi occurring in the potato disease and the cause of their emergence). Bot. Z. 5:314. (In German)

Waterhouse, G. M. 1963. Key to the species of Phytophthora de Bary. Mycol. Pap. 92. CAB International, Wallingford, United Kingdom; Commonwealth Mycological Institute, Kew, Surrey, England.