Cavity spot of carrot (Pythium spp.): Etiology, epidemiology and control
Several Pythium spp. were recovered from cavity spot lesions and pieces of asymptomatic periderm and lateral root scars from tap roots of carrots grown in organic soil in the Holland-Bradford Marsh. Isolates of Pythium violae, P. ultimum and P. irrequlare recovered from lesions caused characteristic cavity spot lesions on carrots grown in infested growth medium and were re-isolated from the lesions. The frequency of Pythium recovery from lesions and asymptomatic root pieces was not closely associated with days after seeding, rainfall or soil temperature. The development of cavity spot throughout the growing season was examined in relations to time (days after seeding), several rainfall parameters and soil temperature at 5 cm depth. Disease incidence and area under the disease progress curve (AUDPC) increased with increasing days after seeding (r2=0.014-0.82). Incidence reached a maximum between 4 August and 27 October (62 to 159 days after seeding) on 24 of 27 disease progress curves recorded over six years. Disease incidence decreased in November or December on four of the 24 disease progress curves. Cavity spot often increased in association with increasing cumulative rainfall and decreasing soil temperatures but effects of these parameters could not be determined because both were highly correlated with days after seeding (r2=0.74-0.99). Increases in incidence followed nine to thirty nine days after a day with rainfall > 20 mm or four consecutive days with total rainfall > 20 mm when the rainfall occurred before 15 October and when soil moisture content was below field capacity (approximately 265% soil moisture by weight). Decreases in incidence followed periods of a minimum of thirteen days where there was no rainfall or rainfall < 5 mm per day. Large AUDPC's occurred in years when soil temperatures were low (16-17.5°C) in the six to eight weeks after seeding and cumulative rainfall was moderate (550 mm per season). The use of the resistant cultivar Six Pak was the most effective method of suppressing cavity spot. Application of metalaxyl as a granular formulation at seeding or as a drench, in combination with mancozeb, applied within six weeks of seeding, was also effective. Metalaxyl plus mancozeb, fosetyl-Al and phosphorous acid reduced disease incidence when applied as a foliar spray 12 or 17 weeks after seeding but were not as effective as an early-season drench application of metalaxyl plus mancozeb. A drench application of metalaxyl plus mancozeb was effective on Six Pak when disease levels were high (3780 incidence days) but not when disease was moderate (1485 incidence days). Application of metalaxyl plus mancozeb to susceptible cultivars such as Chanton and Huron reduced AUDPC to that of untreated Six Pak.