Phytophthora nicotianae var. parasitica
These fungi can infect all parts of the plant. They can cause a damping-off of seedlings, a root and crown rot, a foliar blight and a fruit rot. The symptoms caused by root rot are water-soaked brown lesions on the secondary roots and the tap root that can extend above the soil-line onto the stem. As the disease progresses the smaller roots collapse and decay, and large brown, sunken lesions develop on the larger secondary roots and the tap root. A longitudinal section through the tap root reveals a chocolate-brown discoloration of the vascular system that extends a short distance beyond the lesion. Severely infected plants eventually wilt and die. Infected leaves initially develop water-soaked irregular shaped lesions that quickly collapse and dry. Stem lesions can develop at any level on the stem but are typically found near the soil-line. The lesions are first dark green and water-soaked, and eventually turn dry and brown. As the lesions expand they can completely girdle the stem with the pith becoming brown and collapsing. The fruit symptoms start as grayish-brown water-soaked lesions that can expand rapidly, forming brown concentric rings that resemble a buckeye nut, hence the name. The brown discoloration can extend into the fruit center with the young green fruit becoming mummified, while the mature fruit quickly rots from invasion by secondary organisms.
Conditions for Disease Development
These fungi have a relatively wide host range and can survive in the soil and infested plant debris for at least two years. They can be spread through irrigation run-off and on farm equipment. Initial infection is favored by moderate soil moisture levels and warm (20°C, 68°F) temperatures. Excessive irrigation or rain, in combination with heavy or compacted soils, favors further disease development.
Fungicides can help reduce losses from this disease. In addition, cultural practices that can help reduce losses include using a three-year rotation to non-host crops, improving soil drainage, avoiding soil compaction, using raised beds to improve drainage and using shorter irrigation times to avoid extended periods of soil saturation.