The role of Verticillium spp. in potato early dying syndrome in Australia

© 2014 Dr. Prakash Vijayamma Ramakrishnan NairVerticillium wilt, caused by the soilborne fungi Verticillium dahliae and Verticillium albo-atrum, is a serious disease of potato as well as many other crops. An interaction between Verticillium dahliae and the root lesion nematode Pratylenchus penetrans has been shown to result in early maturation and death of potato crops, which is referred to as Potato early dying (PED) syndrome. However, in Australian potato production systems, very little is known about the role of Verticillium spp. and Pratylenchus spp. in causing PED syndrome. Seed tubers produced by commercial seed growers in Victorian and Tasmanian potato production areas were obtained from 2010 to 2012. A total of 83 seed lots containing tubers from 20 cultivars were assessed for the presence of Verticillium spp. in the vascular tissue of tubers. V. dahliae, V. albo-atrum and V. tricorpus were isolated from the stem end vascular tissue of seed tubers and species identification was confirmed by traditional morphological characters and multigene phylogenetic analysis. Infection by V. dahliae within a seed lot varied greatly and ranged from 0-55%. More than 66% of seed lots tested were not infected by V. dahliae. Over 12% of seed lots tested ranged from 0-5% infection within the seed lots. Only one seed lot had more than 50% of seed tubers infected with V. dahliae. Overall percent infection of seed lots from Victoria and Tasmania were 27.7 (V. dahliae), 8.4 (V. albo-atrum) and 4.8 (V. tricorpus). This was the first report in 47 years of V. albo-atrum infecting potato plants in Australia. Stem-end vascular discoloration of tubers was not correlated with presence of V. dahliae. 11% of vascular discoloured tubers were infected with V. dahliae and 3.3% of tubers without vascular discolouration were infected with V. dahliae, suggesting that tuber stem end vascular discolouration symptoms were not a reliable indication of Verticillium wilt infection. V. dahliae isolates from different geographical locations varied in pathogenicity during infection of potato susceptible cv Shepody, moderately resistant cv Ranger Russet and eggplant cv Black Beauty. The majority of V. dahliae isolates were highly aggressive in potato and eggplant, especially the Tasmanian V. dahliae isolates. Infected plants of cv Shepody inoculated with V. albo-atrum and V. tricorpus showed typical wilt symptoms however, the severity of infection caused by V. tricorpus was substantially lower. This was the first record of the pathogenicity of V. tricorpus in potatoes in Australia. In eggplant, V. dahliae isolates also varied in pathogenicity in terms of disease severity but all isolates significantly (p≤ 0.05) reduced eggplant growth. Internal seed tuber infection of 7 potato cultivars did not significantly contribute to disease development and progeny tuber infection although the pathogen was isolated from symptomless plants. There was no significant difference in the yield of tubers between plants originating from infected and non infected tubers. Nevertheless, infected tubers contributed to an increase in the inoculum level in the soil. There were varietal differences to infection by V. dahliae with infection, transmission and soil inoculum build up; the lowest for the moderately resistant cv Ranger Russet. This was the first study to quantify the level of inoculum of V. dahliae in the soil that originated from naturally infected tubers using molecular techniques. Soil inoculum originating from the decomposition of an infected tuber did not contribute to subsequent disease development in the plants of moderately susceptible Russet Burbank and susceptible Shepody cultivars that were grown in this infected soil. However, there was a positive correlation between high soil inoculum level and isolation of Verticillium from leaf petioles. Tubers of cv Nicola, infected with V. tricorpus, produced very few Verticillium wilt symptoms but the fungus was re-isolated from the petiole of symptomless plants. Only 4% of progeny tubers were infected by the pathogen and the level of V. tricorpus inoculum in the soil at maturity was only 1 cfu/g of dry soil. There was no significant difference in the yield between infected and non infected plants thus indicating that V. tricorpus was a minor pathogen of potatoes. Pot trials proved the synergistic interaction between Pratylenchus crenatus and Verticillium dahliae to cause PED. Potato cv Shepody and eggplant cv Black Beauty, were grown in field soil infested with a high level of the nematode (3 P. crenatus/g of soil) and various concentrations of V. dahliae inoculum. After 8 weeks, potato plants growing in the field soil containing nematodes and 4000 pg DNA/g of soil of V. dahliae inoculum had increased incidence of PED syndrome and significantly reduced tuber yield. There was a threshold level of V. dahliae inoculum of between 300 and 1000 pg V. dahliae DNA/g soil required before infection and colonization occurred in potato plants however, at these inoculum levels there was no visible disease symptoms and no effect on tuber yield. The level of V. dahliae inoculum was quantified in the soil using molecular techniques to assess the threshold level for infection. In eggplant, V. dahliae inoculum concentration of 300 and 100 pg DNA/g of soil in field soil containing 3 P. crenatus/g of soil, resulted in significant reduction in plant height, aerial biomass, and root weight of eggplants. Inoculum levels as low as 85 pg V. dahliae DNA/g of soil and 0.3 P. crenatus/g of soil caused higher disease severity in eggplants. No threshold level of inoculum was found for infecting eggplants