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Verticillium wilt, caused by the soilborne fungi Verticillium dahliae and Verticillium albo-atrum, is a serious disease of potato as well as many other crops. Potato seed tuber surveys (2010 to 2012) from Victoria and Tasmania, Australia identified V. dahliae, V. albo-atrum and V. tricorpus infecting the stem end vascular tissue of seed tubers. The species were identified by traditional morphology and phylogenetic analysis of the ITS region. Isolation of V. dahliae within a seed lot varied greatly and ranged from 0 to 55%. Verticillium spp. were isolated from the stem-end vascular tissue of tubers from seed lots from Victoria and Tasmania with an overall percent infection of 27.7 (V. dahliae), 8.4 (V. albo-atrum) and 4.8 (V. tricorpus). Verticillium dahliae was isolated from 11% of tubers with discoloured stem-end vascular tissue and 3.3% of tubers without stem-end vascular decolourisation suggesting that tuber stem end vascular discolouration symptoms were not a reliable indication of Verticillium wilt infection. In glasshouse trials, V. dahliae isolates from different geographical locations varied in pathogenicity during infection of susceptible potato 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 compared to highly aggressive isolates of V. dahliae. In eggplant, V. dahliae isolates also varied in pathogenicity in terms of disease severity but all isolates significantly (p ≤ 0.05) reduced eggplant growth.

Colonization of stems, symptom development, yield, progeny tuber infection, quantification of soil inoculum and contribution of soil inoculum on subsequent disease development by Verticillium dahliae, causal agent of Verticillium wilt of potato was evaluated in the glasshouse using naturally infected and non-infected seed tubers of seven cultivars in two trials. Internal seed tuber infection of seven potato cultivars did not contribute to severe disease development although the pathogen was isolated from symptomless plants. Susceptible cultivars Atlantic, Trent and Russet Burbank had the highest incidence of petiole infection while the moderate resistant cv Ranger Russet had the lowest incidence at 65 days after planting. Progeny tuber infection in various cultivars after harvest ranged from 0 (cv Ranger Russet) to 13 % (cv Atlantic). Planting infected tubers contributed to V. dahliae inoculum in the potting soil. Mean levels were highest from cv Russet Burbank (70 pg DNA/g of soil; range from 6 to 149) while the least was recorded in cv Ranger Russet planted soil (11 pg DNA/g of soil; range from 3 to 31). Soil inoculum originating from naturally infected tubers did not contribute to subsequent foliar disease development in Russet Burbank and Shepody cultivars that were grown in this infected soil. However, V. dahliae was isolated from the petiole of some symptomless plants. Progeny tubers were completely free from V. dahliae. Tubers of cv Nicola, infected with Verticillium 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 of tubers between plants originating from infected and non-infected tubers.

Measurement of pathogens on seed tubers is essential for informing likelihood of subsequent potato disease. Here we utilized quantitative PCR assessment of pathogen DNA and visual assessment of disease to measure seed tuber inoculum and used this to model development of disease in potato grown in pathogen-free soil. Analysis by recursive partitioning and modeling using receiver operating curves indicated both abundance of Rhizoctonia solani AG3 and Streptomyces scabies DNA, and disease symptoms associated with these pathogens on seed tubers could predict subsequent disease in progeny tubers and for R. solani, stolons. In contrast, abundance of Spongospora subterranea DNA and disease symptoms on seed tubers were not consistently associated with powdery scab in progeny tubers. The relationship between S. subterranea DNA and seed tuber symptoms on root galling was stronger. Symptomless seed tubers that carried high levels of S. subterranea DNA were also associated with greater root galling than those with low pathogen DNA levels. There was a modest association between root galling and powdery scab in progeny tubers. These results highlight the importance of using certified seed tubers, and demonstrate a statistical tool for measuring the impact of seed tuber-borne inoculum.