| Summary: | In recent years changes in the occurrence of the first potato late blight (Phytophthora infestans) symptoms have been noted. Observations of potato crops have showed that occurrence of the blight symptoms is sometimes on the stem instead on the leaves. The aim of the study carried out in 1995-2001 was to determine occurrence and harmfulness of the stem late blight infections. Survey of many potato crops (490 potato fields) made in 1997-2001, around Poland enabled assessment of the incidence of stem blight. Average 69,1% of observed crops were affected with stem blight. The stem blight incidence was not connected with the rate of the leaf form of late blight developing on the foliage. Stem form of the disease has appeared more often in the years with less rainfall during June and July. Its occurrence was influenced by micro climate inside potato canopy. Glasshouse experiments confirmed the importance of stem lesions in decreasing tuber yield (11,2-42,9%) and increasing tuber infections. Results have shown that tubers contaminated with spores were the least probable source of the stem form of late blight.
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