Development of a disease suppressive soil for managing cavity spot of carrots

Cavity spot of carrots is generally managed by crop rotation, fungicides, and fumigants. In the last several years we studied the ability of incorporated green manures to create a disease suppressive soil by elevating microbial activity. The detrimental effects of Pythium species were significantly reduced by incorporating manures, but the beneficial effect of the elevated soil organic matter was short-lived, lasting only as long as the newly incorporated organic matter was decomposing. In addition to direct measurements of Pythium populations, total microbial activity was directly assessed by fluorescein diacetate hydrolytic activities. In general, culturable populations of bacteria and fungi did not significantly vary between treatments at multiple time points. However, because organisms cultured in the laboratory may represent just a fraction of the organisms found in soil, we used Illumina DNA sequencing to measure microbial community diversity. There were some significant differences among treatments. Soil amended with composted green manure had a greater diversity of bacteria relative to all other treatments, and the diversity of bacteria in plots treated with metam was significantly lower than bacterial diversity in all other treatments. Pythium sensitivity to mefenoxam was evaluated. Sixty-eight Pythium isolates were obtained from carrot roots with cavity spot symptoms in 2012-13. All Pythium isolates were classified into four species based on morphological characteristics and DNA sequences (P. violae, P. sulcatum, P. ultimum, and P. irregulare). Pythium violae was the primary cause of cavity spot in 2012-13, consistent with historical reports. Most Pythium spp. were sensitive to mefenoxam. However, three isolates of P. irregulare were completely insensitive to mefenoxam.