Phytophthora rot is caused by several species of Phytophthora, a group of water moulds that thrive in wet, poorly drained soil. Depending on the crop and species involved, the pathogen may attack roots, crowns, stems or fruit.Infected roots and crowns become brown to black and begin to rot, restricting the plant’s ability to take up water. Above ground, plants may wilt even though the soil is wet, become stunted or suddenly collapse. The pathogen produces spores that move readily through water and saturated soil and can persist in contaminated soil and plant debris.

How to Fix It
Remove badly infected plants and dispose of them. Avoid moving contaminated soil to other parts of the garden, and clean soil from tools and equipment before using them elsewhere. There is no treatment that will restore badly rotted roots or crowns.Drainage is the big one. Avoid planting susceptible crops in areas that remain wet, improve drainage where possible, use raised beds if necessary, and don't overwater. Rotate away from susceptible crops, although rotation alone may not eliminate Phytophthora because some species survive in soil for years and have broad host ranges.Some biological products containing organisms such as Trichoderma or Bacillus species may help suppress certain Phytophthora diseases when used preventively, but they are not a cure. Keeping roots out of persistently wet soil is the most important control for the home garden.