Phytophthora Isolation
Practical guide to detection and identification of Phytophthora
Leaf blight
A number of Phytophthora species cause leaf blight. These include: P. infestans on potato and tomato; P. palmivora on a large number of tropical fruit species including rubber, durian and macadamia; and P. colocasiae on taro. These blights on leaves are first seen as small flecks but within 3-5 days they expand to produce large lesions. Initially, infected tissue is water soaked but becomes necrotic (brown or black) in a few days. Often the lesions are surrounded by a halo of light green tissue. Spores appear as white velvety growth at the edge of the lesions, primarily at the underside of the leaf. It is this white growth that distinguishes Phytophthora leaf blight from several other foliar diseases. Often large amounts of sporangiospores are produced as 1-4 sporangiophores extend from the stomata at the underside of the leaf and produce large numbers of sporangiospores which can either be airborne under relatively dry conditions or differentiate into numerous zoospores under wet and humid conditions. These zoospores can encyst and form new lesions on the same leaf or plant and can spread to neighbouring plants through leaf to leaf contact.
Tuber and corm rot
Tuber infections are characterized by patches of brown to purple discoloration on the potato skin. Cutting just below the skin reveals a dark reddish brown, dry corky rot. Heavy infection can give rise to total loss of the tubers. Light infections can occur and are difficult to detect.
Media and antibiotics for isolation of Phytophthora from diseased plant tissue and soil
The Oomycetes are not true fungi, and therefore special techniques are required for their isolation. Most species of Phytophthora grow rather slowly in vitro compared with saprophytic fungi and bacteria. In addition, bacterial populations need to be kept low because they may suppress the growth of Phytophthora by direct competition, by antagonism caused
Leaf blight
A number of Phytophthora species cause leaf blight. These include: P. infestans on potato and tomato; P. palmivora on a large number of tropical fruit species including rubber, durian and macadamia; and P. colocasiae on taro. These blights on leaves are first seen as small flecks but within 3-5 days they expand to produce large lesions. Initially, infected tissue is water soaked but becomes necrotic (brown or black) in a few days. Often the lesions are surrounded by a halo of light green tissue. Spores appear as white velvety growth at the edge of the lesions, primarily at the underside of the leaf. It is this white growth that distinguishes Phytophthora leaf blight from several other foliar diseases. Often large amounts of sporangiospores are produced as 1-4 sporangiophores extend from the stomata at the underside of the leaf and produce large numbers of sporangiospores which can either be airborne under relatively dry conditions or differentiate into numerous zoospores under wet and humid conditions. These zoospores can encyst and form new lesions on the same leaf or plant and can spread to neighbouring plants through leaf to leaf contact.
Tuber and corm rot
Tuber infections are characterized by patches of brown to purple discoloration on the potato skin. Cutting just below the skin reveals a dark reddish brown, dry corky rot. Heavy infection can give rise to total loss of the tubers. Light infections can occur and are difficult to detect.
Media and antibiotics for isolation of Phytophthora from diseased plant tissue and soil
The Oomycetes are not true fungi, and therefore special techniques are required for their isolation. Most species of Phytophthora grow rather slowly in vitro compared with saprophytic fungi and bacteria. In addition, bacterial populations need to be kept low because they may suppress the growth of Phytophthora by direct competition, by antagonism caused