Identification of resistance and use of soil amendments to control Verticillium wilt of potato in Australia

© 2016 Dr. Veradina Nugrahaningtyas DharjonoVerticillium wilt of potato caused by Verticillium dahliae is a widespread problem throughout the potato production areas of the world. Verticillium wilt is a soil borne disease and has mainly been controlled through application of soil amendments and soil fumigation. Very few potato cultivars have resistance to V. dahliae. In Australia, resistant cultivars have yet to be identified and soil amendments have not been implemented as an integrated control method. Verticillium wilt disease was successfully established in the susceptible cultivar Russet Burbank in glasshouse bioassays using either spore suspension or microsclerotia (MS) as inocula. Dipping potato seedling roots in a spore suspension of V. dahliae prior to planting in potting mix was more effective than planting seedlings in potting mix inoculated with artificially produced MS. In general, disease incidence and severity increased as inoculum density increased for both inoculation methods. High concentration of 105 spores/ mL caused premature senescence and plant death, whereas high concentration of MS 500 CFU/ g soil caused severe disease symptoms (disease rating 4). With the spore suspension inoculation method, an inoculum threshold effect was identified whereby a minimum inoculum concentration of between 102 and 103 spores/ mL was required to cause infection and subsequent disease development. However, with MS inoculation method the minimum amount of inoculum to cause infection was not identified. There was no correlation between the methods used to quantify V. dahliae in the potting mix and soil. The qPCR method (pg DNA/ g potting mix) resulted in detection of higher levels of V. dahliae inoculum especially at the end of the bioassays (12 weeks) compared to the dry plating and counting of viable MS (CFU/ g potting mix). The qPCR method probably detected DNA from mycelia in the potting mix and roots; as well as viable, dead or dormant MS in the potting mix whereas the dry plating method only detected viable MS in the potting mix. Nevertheless, the dry plating method was less expensive than qPCR, only measured viable MS and was conducive for screening large number of soil samples, thus was the preferred method to quantify the inoculum in the soil and potting mix. There was higher numbers of viable MS in the inoculated potting mix (CFU/ g potting mix) at planting compared to the number of viable MS in the inoculum (CFU/g soil). This was most likely due to fragmentation of MS into smaller propagules during the action of mixing MS in the soil prior to transfer of the inoculated potting mix to pots. The reduction in actual number of viable MS (CFU/ g potting mix) at harvest was presumably due to a decrease in MS viability over the 12 weeks of trials. Glasshouse screening trials identified cultivar Denali as moderate to highly resistant and cultivars Catani and Desiree as moderately resistant to V. dahliae. These trials used both spore suspension and MS inocula. Cultivar Denali showed a disease rating of 0 (no infection) to 2 (moderate disease symptoms) with a low incidence of infection. Cultivars Nicola, Russet Burbank, Shepody and Trent were susceptible to V. dahliae in root dip inoculation. Cultivars Coliban, Pike, Kennebec, Simcoe, Ranger Russet and Atlantic showed inconsistent host reactions in the three trials. Cultivars Pike and Kennebec were resistant in the first trial and susceptible in the second trial. In contrast, cultivars Simcoe and Ranger Russet were susceptible and moderately resistant in the first and second trials respectively. This inconsistency in host reactions may have been due to the fact that the trials were planted at different times of the year where the light and temperature conditions varied. A microcosm assay was developed to assess the efficacy of soil amendments to reduce MS viability. The addition of both 1% and 10% sulphur, brown coal and blood and bone meal (BBM) significantly reduced the pH of the soil, changed the biodiversity and abundance of microbial communities in the soil, and reduced the viability of MS. Soil amendments at 1% lowered the soil pH, whereas at 10% sulphur and brown coal reduced soil pH, and BBM increased soil pH. The change in the microbial communities may have resulted in selection of microorganisms that directly inhibited MS viability however, further research is required to identify specific inhibitors and their mode of action. Microbial abundance and biodiversity in the microcosm assays was also affected by the soil moisture. Regular addition of water to keep the soil at 15% soil moisture resulted in a greater biodiversity of microbial communities during the 8 weeks of the assays compared to the lower diversity of microbial communities in the soil with no additional water added during the course of the microcosm assay. Nevertheless, the reduction in MS viability was similar in soils with and without water adjustment. Verticillium wilt disease was established in eggplants inoculated with the potato V. dahliae isolate TASVd27 in glasshouse bioassays using both spore suspension at a minimum of 102 spores/ mL and soil MS inocula at a minimum concentration of 500 CFU/ g field soil or potting mix. Similar to potato plants, disease severity increased as inoculum concentration increased. A high inoculum concentration of 105 to 106 spores/ mL caused severe disease symptoms. Root dip inoculation method also caused more severe disease symptoms in eggplants than the MS inoculation method. Eggplants inoculated with the potato isolate of V. dahliae at 106 spores/ mL had 60% reduction in growth and 91% reduction in dry weight 9 weeks after inoculation. In a glasshouse trial to assess the efficacy of soil amendments to reduce the viability of MS, 1% brown coal reduced the incidence of infection in eggplants in field soil inoculated with MS at 500 CFU/ g soil however, there was no reduction in disease severity of infected plants. Sulphur at 1% caused manganese and aluminium toxicity and although the incidence of infection in inoculated plants decreased, there was an increase in disease severity, probably as a result of an interaction between the sulphur amendment and V. dahliae. Although further research is required to assess cultivar resistance and efficacy of soil amendments in field situations to reduce Verticillium wilt, the outcomes of this study provides the potato industry with options to manage Verticillium wilt disease