Strategies for control of white blister disease in radish (Raphanus sativus L.) seed crops: A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science at Lincoln University

Abstract

New Zealand produced radish seeds are exported to Continental Europe and Asia, with an export value exceeding $25 million in 2018. However, in recent years, white blister disease caused by the obligate biotrophic parasite Albugo candida has significantly reduced seed yields in radish crops. Growers currently use a fungicide seed treatment and multiple fungicide foliar sprays in an attempt to control the disease, but the effectiveness of this programme is variable. Albugo candida is seedborne and oospores can be readily detected from the seed coat. However, whether the pathogen is present within the plant allowing vertical transmission via seed is not known. The presence of A. candida inside radish plant tissues was evaluated using PCR with one primer pair PKG-F and PKG-R. Pathogen DNA extraction was carried out using two methods: Chelex® resin and a Genomic DNA Mini Kit (Gene Aid, Taiwan). The latter was able to allow detection of A. candida DNA from all the types of radish tissue tested. Albugo candida was not detected inside radish plant tissues or seeds and contrary to previous reports, this strongly suggests the pathogen only exists on the plants externally, and on the seed coat. PCR was also used to identify A. candida on infected tissue of turnip and the common weed shepherd’s purse. Whether the same race of A. candida occurred on all three plant species needs to be determined. Fungicide seed treatments (Ridomil Gold MZ WG, Thiram and Iprodione) and non-chemical alternatives including hot water (50°C for 15, 20, or 25 mins) and Trichoderma atroviride (strains LU132 and LU140) were evaluated for their ability to reduce the transmission of A. candida from seed to seedling in two glasshouse experiments, one of which was overhead watered, and the other soil only watered. None of these treatments reduced disease transmission in either experiment. However, the disease incidence and severity were much higher in overhead watered plants, confirming that current irrigation methods for seed crops are contributing to disease spread and intensity within the crop. A field trial in a radish seed crop was conducted in the 2018-2019 season, with eleven fungicide treatments (single products and combinations of products over the growing season). White blister disease severity was reduced by Ridomil Gold MZ WG, Cobra, Foschek plus Metalaxyl-M, Ranman plus Mancozeb plus Amistar followed by Ranman plus Mancozeb plus Seguris Flexi, then Ranman plus Mancozeb, then Ridomil Gold plus Pristine and finally Ridomil Gold, or Ranman plus Mancozeb followed by Ranman plus Mancozeb plus Seguris Flexi then Ranman plus Mancozeb plus Amistar, then Ridomil Gold MZ WG plus Pristine and finally Ridomil Gold MZ WG, but no treatments reduced the percentage of stagheads, infected racemes or infected pods per plant. Seed yield and quality were not increased by any of the treatments. Seedborne inoculum is the most important source for establishment of white blister in New Zealand radish seed crops. Sourcing disease free seed, or finding on effective method for removing oospores from the seed coat before sowing, may be the best management method for reducing the impact of this pathogen in New Zealand. In the laboratory surface sterilising seeds using 5% sodium hypochlorite killed all oospores. Whether this can be used for an entire seed lot requires investigation.