Diseases of Vegetable, Ornamental and Spice Crops

Exercise 2: DISEASES CAUSED BY PHYTOPHTHORA SPP.

a) Late blight of potato/tomato (Causal organism Phytophthora infestans (Mont.) de Bary

Kingdom : Chromista
Phylum : Oomycota
Class : Oomycetes
Order : Peronosporales
Family : Pythiaceae
Genus : Phytophthora
Species : infestans

Symptoms: On leaves, the disease appears as pale green, irregular spots on the tips and margins, which in moist weather enlarge rapidly with central tissue turning necrotic and dark brown or black. On the ventral surface of the leaves, a white downy growth of the fungus appears around the dead areas. Brown streaks also develop along the stems. The infected tubers show irregular, small to large, slightly depressed areas of brown to purplish skin which extend deep into the internal tissue of the tuber. On tomato fruits, symptoms appear on fruits as olivaceous greasy spots, which gradually cover the entire fruit resulting in fruit rot.

Mycelium: Well developed, branched, coenocytic, hyaline, inter and intra cellular, haustoria formed.

Asexual reproduction: Sporangium lemon shaped, papillate, formed terminally on branched sporangiophore (Fig. 1) but become intercalary when sporangiophore again moves forward giving zigzag shape with bulbous base. Zoospores produced in sporangium. Chalamydospores round and dark brown.

Sexual reproduction: Oogamous, by gametangial contact, oospores smooth, thick walled, round (Fig. 1) and dark brown.

Fig.1. Microscopic details of Phytophthora

b) Buckeye rot of tomato (Causal organism Phytophthora nicotianae B. de Hann var. parasitica (Dastur) Waterhouse)

Kingdom : Chromista
Phylum : Oomycota
Class : Oomycetes
Order : Peronosporales
Family : Pythiaceae
Genus : Phytophthora
Species : nicotianae
variety : parasitica

Symptoms: Appear as water soaked light brown discoloured spots on green fruits that increase readily showing concentric dark brown rings slightly resembling the markings on a buckeye nut. The lesions rapidly enlarge and in 3-4 days whole of the fruit surface turn dark brown and are soft to touch. White flocculent superficial growth of the fungus consisting of sporangia develops profusely on the diseased fruits in warm and humid weather.

Mycelium: Hyaline, coenocytic with finely granular protoplasm, branching typically at right angles with slight constriction at the point of branching.

Asexual reproduction: Sporangiophores arise from hyphal threads and produce sporangia generally at their tips in nature while in culture, the sporangia are terminal as well as intercalary. Sporangia are ovoid to globose in shape, but elongated to barrel shaped sporangia are also occasionally found. Sporangia germinate by producing zoospores and occasionally by germtube. Chlamydospores are smooth, globose, and slightly yellowish with thick brown walls.

Sexual reproduction: Oogamous, by gametangial contact. Anthredia are amphigynous, spherical or oval and oogonia are rough, thick walled, yellow brown. Oospores are aplerotic with thick wall.


c) Leaf blight and fruit rot of bell pepper (Causal organism Phytophthora nicotianae Breda de Hann var. nicotianae Waterhouse)

Kingdom : Chromista
Phylum : Oomycota
Class : Oomycetes
Order : Peronosporales
Family : Pythiaceae
Genus : Phytophthora
Species : nicotianae
Variety : nicotianae

Symptoms: Appear as water soaked to dull or faded green areas differing slightly from the color of the normal healthy leaf tissue rapidly enlarges covering entire leaf, resulting in premature leaf fall. Infection on green fruits begin as water soaked, dull green spots that expand rapidly to encompass the entire fruit, which later become mummified. Whitish grey mould appears on the rotten fruits under humid conditions. Completely rotten fruits may fall down on the ground. Symptoms on collar region of adult plants appear as water soaked dark brown areas engirdling the collar region at the point of contact with soil line.

Mycelium: Hyaline, branched and coenocytic with branches at right angles.

Asexual reproduction: The sporangia are hyaline, non-pedicellate, and globose to subglobose with a prominent papilla at the apex. The sporangia germinate only by zoospores and produce 15-20 zoospores.

Sexual reproduction: Oogonia are hyaline and globose. The oogonial stalk is encompassed by a persistent amphigynous anthredium. The oospores are thick walled golden brown in colour.