Pythium sulcatum

Pythium sulcatum causes diseases predominately on members of the apiaceous family, the most common being the carrot. This pathogen causes cavity spots which are sunken, spherical to elliptical oriented, brown-black spots across the breadth of the surface of the carrot.^[1] These cavity spots are normally 1-10 mm in width and are often surrounded by a pale halo.^[1] The lesions enlarge as the roots mature, often most prominent around the time the crop is harvested. This pathogen is believed to be native of the apiaceous family that eventually became pathogenic to carrots.^[2] P. sulcatum is also seen to cause damping off, taproot dieback, root rotting, stunting, and forking of carrots, with cavity spots being the most common of these diseases.^[3][4] Because carrots are a root vegetable the cavity spots are only apparent after the carrot is harvested and washed.^[1]

Pythium sulcatum can cause disease in both mineral and organic soils.^[5] The pathogen prefers wet to moist soils with a relatively acidic pH content, as a pH of 6.8 is ideal.^[1] However, the pathogen has been seen to grow in acidic pH of 3-5 and basic pH of 9-10.^[6] P. sulcatum prefers a relatively warm temperature, with 20°C–28°C being the most common, 25°C being optimal.^[1] This is why the pathogen is most severe in summer and autumn-harvested crops.^[3] The pathogen overwinters in the form of an oospore, which can survive several years in the soil and inoculates the host when temperatures become optimal. In heavy moisture and wet soil, the pathogen can also release zoospores that can further increase its population up to 1000-fold.^[3][4] Flooding of soil by heavy rains as well as poorly drained soils have been shown to increase cavity spot disease development.^[5]