Biological control of Sclerotinia sclerotiorum by film-coating Coniothyrium minitans on to sunflower seed and sclerotia

Abstract

Conidia of the biological control agent, Coniothyrium minitans, were applied to uninfected and S. sclerotiorum-infected sunflower seeds and to sclerotia of two isolates of S. sclerotiorum by polymer film-coating, using a fluidised-bed seed treater. Rates of 5 x 106 and 7-14 x 106 conidia per g of seed and sclerotia were obtained, respectively. The activity of C. minitans was compared to treatments with the fungicides thiram + fenpropimorph and iprodione. In agar plate tests, C. minitans increased seed germination and decreased recovery of S. sclerotiorum from S. sclerotiorum-infected seeds. However, in seedling tests carded out in a potting compost-soil mix with S. sclerotiorum-infected seeds, C. minitans failed to increase survival of seedlings. A high and low rate of thiram + fenpropimorph and a high rate of iprodione film-coated onto S. sclerotiorum-infected seeds gave consistently better results than C. minitans in these tests. The film-coating process slightly decreased the germination of C. minitans conidia recovered from sunflower seeds and sclerotia or S. sclerotiorum, but after storage for 1 year at 10°C C. minitans grew from more than 97% of seed and sclerotia, and the number of colony forming-units per g seed or sclerotia had declined by less than 1 log10 unit. In agar plate tests, all fungicide treatments were more effective than C. minitans in delaying myceligenic germination of film-coated sclerotia, although for sclerotia of isolate JN1, all film-coatings were effective in decreasing the weight of sclemtia subsequently produced. With isolate SB, only the C. minitans treatment reduced subsequent sclerotial production. In glasshouse and field pot bioassays with sclerotia buried in soil for 20 and 34 weeks, respectively, C. minitans completely suppressed apothecial production and no sclerofia were recovered from the C. minitans treatment pots. C. minitans spread to infect sclerotia in other treatment pots. In these bioassays, both fungicides were less effective than C. minitans, even though apothecial production was almost completely inhibited and less than 10% of sclerotia were recovered.