Viscous Exudate Research Articles

Reliability of the stem-ooze test for field identification of potato ring rot

The field diagnosis of potato ring rot often is complicated by other vine disturbances such as cultivation- and drought-wilting, tip- and hopper-burn, early- and late-blight, other wilt diseases, frost killing, and the like. A serviceable test under such conditions consists of cutting the suspected stem near the point of original seed-piece attachment. squeezing the stem at the cut, and looking for a pearly, milky, viscous exudate at a locus or line between the woody vascular ring and the pith. In the case of plants infected through the seedpiece,Corynebacterium sepedonicum occurs in the new plant first at the point of attachment to the mother seedpiece; only later are the bacteria demonstrable in other parts of the plant such as the tubers and the aerial vines. Four hundred and thirty-two potato plants, representing II varieties, grown under two different field environments, and planted and harvested on two different dates, were used to determine the degree of relationship between presence of ring rot (indicated by positive Gram smears) and the occurrence of external vine wilt and of stem ooze. The percentage of plants which were correctly diagnosed by external vine symptoms was 85.6. The percentage of disagreement due to few bacteria resulting in positive Gram smears but insufficient to produce wilting, amounted to 12.8. The percentage representing erroneous judgment as to what constituted ring-rot wilt was 1.6. The percentage of plants which were correctly diagnosed by stem-ooze tests was 97.2. The percentage of disagreement due to few bacteria resulting in positive Gram smears but insufficient to produce stem ooze, amounted to 1.9. The percentage representing erroneous judgment as to what constituted ooze amounted to 0.9.