A comparison of tuber slice and whole tuber tests for the assessment of potato resistance to tuber blight Phytophthora infestans (Mont.) de Bary

The resistance of potato tuber tissues to colonization by <i>Phytophthora infestans</i> was compared in 10 selected cultivars in two trials: on tuber slices and whole tubers with cut rose ends. When resistance to sporulation was used as the discriminating criterion, the selected cultivars segregated in a distinctly different order than when arranged according to other resistance components. Both the tuber slice test and the whole tuber test allowed resistance to sporulation to be assessed in the tested group of cultivars. The results obtained in both trials were reproducible and both criteria, i.e. the percentage of mycelium-covered area as well as sporulation intensity were equally useful for the assessment. No significant influence of the fungal growth though the tuber slice tissue was observed on the assessment of sporulation in comparison to the whole tuber test where the fungus grew directly on the wounded tissue after inoculation. Assessment of the percentage of the area of the tuber slice exhibiting visible necrosis was found to be useless due to the lack of correlation with two criteria of resistance to colonization used in the whole tuber test, i.e. the rate and depth of penetration. The most useful parameter of the whole tuber test appeared to be assessment of the depth of penetration of the tuber tissues. This parameter showed the least variability and good correlation with other criteria used to evaluate resistance to colonization.

Development of Tuber Blight (Phytophthora infestans) on Potato Cultivars Based on In Vitro Assays and Field Evaluations

Tuber blight may result from infection of wounded or unwounded potato tubers exposed to sporangia from foliar blight, soil, or blighted tubers. However, there are limited data on the prediction of tuber blight in field or storage environment based on in vitro assays. To assess this relationship, potato cultivars with foliar blight resistance (R-genes) and general resistance were evaluated for tuber blight incited by Phytophthora infestans (US-1) based on wound-induced and unwounded tuber inoculations. Surface lesion diameter, lesion depth, and frequency distribution of blighted tubers were assessed in in vitro assays and tuber blight incidence determined in field experiments. Significant differences (P < 0.05) in lesion diameter and depth were recorded among cultivars. Surface lesion diameter, depth, and index ranged from 5 to 40, 2 to 16.3, and 15 to 656 mm, respectively, in wound-inoculated tubers. In nonwounded tuber assays, the incidence of blighted tubers ranged from 0% to 8.7% in both years. Tuber blight infection of potato cultivars varied between years in field studies. Although tuber infection differed among cultivars, the frequency of blighted tubers had a normal statistical distribution irrespective of R-genes, implying that foliar resistance may have limited effect on tuber blight occurrence based on in vitro experiments. Prediction of tuber blight based on inoculation assays can be effectively used to estimate and manage blight development in storage environments.