Increased bulb yield following seed coating of radish (Raphanus sativus L.) with selected isolates of Trichoderma species in Rhizoctonia solani infected soil

Abstract

Abstract: Red radish (Raphanus sativus) is highly susceptible to the soil-borne fungus Rhizoctonia solani, which can cause severe crop losses. In a glasshouse experiment, untreated seeds of radish cvs. French Breakfast and Red Round were grown in potting mix where R. solani inoculated wheat-bran was added at rates of 0.25, 0.5 and 1.0 g per 100 g potting mix. Seedling emergence was reduced by one third and two thirds respectively by the two higher inoculum rates, and final plant numbers were ca. 20%, 50% and 80% less than in the uninoculated control. The ability of Trichoderma spp. to increase radish yields by limiting the damage caused by R. solani has long been known but has not been evaluated in New Zealand. Inoculum of each of four Trichoderma spp. isolates LU132 (T. atroviride), LU785 (T. hamatum), LU1437 (T. harzianum) and LU1358 (T. polysporum) was prepared in sterile wheat-bran and 0.5 g wheat-bran was added per 100 g potting mix. In a second glasshouse experiment, R. solani (0.25 g inoculated wheat-bran) was added per 100 g potting mix before untreated seeds of both radish cultivars were sown. Potting mix without either R. solani or Trichoderma served as the control. Maximum seedling emergence did not differ among the treatments for cv. French Breakfast, but was increased by the presence of either isolate LU132 (T. atroviride) or LU1358 (T. polysporum) for cv. Red Round. The presence of isolate LU1347 (T. harzianum) in the potting mix significantly increased plant survival in both cultivars. Each of the four Trichoderma isolates reduced the percentage of diseased plants with isolate LU132 (T. atroviride) providing the strongest response. In a third glasshouse experiment, Trichoderma treated seeds, thiram fungicide treated and untreated seeds of both radish cultivars were sown in naturally R. solani infected soil. The same treatments were used in a field trial at a site known to be infected by R. solani. In the third glasshouse experiment, seed treatment with Trichoderma isolates LU1347 (T. harzianum), LU1358 (T. polysporum) and LU785 (T. hamatum) significantly increased bulb fresh weight in cv. Red Round, but no treatments increased bulb fresh weight in cv. French Breakfast. In the field experiment, bulb yield for the thiram seed treatment did not differ from that of the untreated control. However, seed treatment with isolate LU785 (T. hamatum) increased subsequent bulb yield by 96% for both cultivars, and seed treatment with isolate LU132 (T. atroviride) or isolate LU1358 (T. polysporum) also significantly increased bulb yield (by 85% and 60% respectively) in cv. French Breakfast. A possible explanation for this result was sought by undertaking a fourth glasshouse experiment for radish cv. Red Round only. In this experiment, all four Trichoderma spp. isolates more than doubled bulb yield by producing not only a greater number of bulbs but also larger bulbs than the untreated control. Trichoderma seed coating may provide an alternative to fungicide seed treatment for radish production.