The inflorescence and mature seeds of sorghum plants infected bySclerospora sorghi were subjected to histological studies. The mycelia of the pathogen were detected in the inflorescence axis, ovary wall, anther wall and endosperm of the seed. Infected anthers contained shrunken pollen grains. Oospores were detected in the glumes and pericarp of mature seeds. However, there was no trace of the pathogen in the embryo either as mycelium or as oospores. Upto 80% of the seedlings showed systemic infection when samples of seeds with glumes from systemically infected plants were sown in sterile soil. 1–4% infection was observed in seeds sown without the glumes, probably because the oospores contained in them were eliminated. Although mycelium and oospores were found in the seeds, in proper sun-dried seeds they may not remain viable.

Downy mildew infected ragi plants exhibits a wide range of symptoms. Histopathological studies of the diseased plant have revealed that the fungus mycelium is present in root, stem, floral parts and seed causing morphological and anatomical changes. The mycelium develops profusely in the sub-stomatal spaces and from this the sporangiophores emerge through the stomata. In the leaf tissue invaded by the fungus, the chloroplasts and leucoplasts are either few or absent. Cells of the mesophyll in the case of the diseased leaves are distorted. Sometimes, the cells of the invaded tissue dissolve and the mycelium or sex organs occupy the space thus created. In the leaf tissue, the intercellular spaces surrounding the vascular sheaths are the primary centres of mycelial development. The sex organs are mostly confined to the vicinity of the vascular bundles. In the infected leaf, very few epidermal hairs are developed.

Preheating and heated teliospore extracts have been found to be stimulatory to germination of the teliospores of ten different smut fungi used in this study. The effect is mostly on the percentage of germination.

Maximum per cent of spore germination was observed at higher concentrations of glucose and sucrose solutions. The percentage of germination of spores of the species included was more in sucrose than in glucose. Most of the vitamins of B-complex groups stimulated the initiation of germ tube.

Many of the growth regulators tried, except 2,4-dichlorophenoxy acetic acid have a stimulatory effect on teliospore germination. Higher percentage of teliospore germination was noticed in the case of gibberellic acid, followed by indole 3-acetic acid, beta-indole butyric acid and alphanaphthalene acetic acid.

Ethylenediamine tetraacetic acid, furfural, fumaric acid, oxalic acid and citric acid were also stimulatory, in that order.

Sclerospora sorghi has been cultured in callus tissues ofSorghum vulgare maintained on a modified White’s medium. Cultures of healthy and diseased calluses were initiated from surface sterilised explants of stem apices and axes of inflorescences of healthy and diseased plants, and subcultured. The fungus developed internally in the callus and also spread over its surface as a white cottony mycelium. Healthy callus tissue and the sorghum seedlings were infected by placing them in contact with the mycelium produced on diseased callus. Mycelium spread from infected callus onto the culture medium, but remained viable only if physical contact with the callus was maintained. Only mycelium and oogonia were observed in infected callus. The number of nuclei in the oogonia varied. Some oogonia were abnormal both in size and shape. None of the oogonia developed oospores.

Peronosclerospora sorghi, produced a maximum of 10,800 conidia/cm² of diseased sorghum leaves at 100% relative humidity but only about 3600 conidia at 85% relative humidity underin vitro conditions. The sporulation was totally inhibited at 80% relative humidity and below. Infected sorghum leaves kept in darkness after completion of the previous crop of the spores, did not sporulate in continuous darkness even at the optimum relative humidity and temperature. Optimum temperature for sporulation is 21–23° C, 31° C and 30° C are minimum and maximum respectively. At 26° C and above, conidiophores were malformed and produced only a few conidia. For conidial germination, 21–25° C were optimum while at 13° C conidial germination was as low as 52%. At 32° C, 80% germination was recorded but 35° C and above no germination occurred. After inoculation with conidial suspension, a minimum of 3 hr moisture was essential to induce systemic infection.

The carpels of pearl millet (Pennisetum typhoides) are infected bySclerospora graminicola. Vegetative mycelium present in the diseased mother plant infects the carpel through the stalk of the spikelet. This process may or may not cause hypertrophy. Zoospores infect the carpel through the stimga and style, without inducing hypertrophy. Infection process leading to the establishment of downy mildew mycelium in the carpellary tissue and its implications are discussed.

Age of sorghum plants is important in the development of downy mildew disease incited byPeronosclerospora sorghi. Plants inoculated just after emergence and up to 4–5 leaf stage are highly susceptible. In plants inoculated after 6–7 leaf stage, systemic symptoms were not observed but only local lesions appeared. Conidial concentration of 40/seedling brings about 100% infection if the host seedlings are inoculated through root. Systemic infection occurs in 10 and 22 days depending upon the conidial concentration. Roots of the seedlings inoculated with 1000 conidia/seedling get infected earlier. Mature conidia are highly infective compared to immature or old conidia. Soil and seed-borne inoculum can initiate both systemic and local lesion type of symptoms at any growth stage of the host plant in addition to air-borne conidia. Late expression of systemic infection can result both from air-borne conidia and oospore present in the soil or seed.

In the present study an attempt has been made to establish the fate of sporangia ofSclerospora graminicola (Sacc.) Schroet deposited in the soil. A technique has been standardised to demonstrate the germination of sporangia and the viability and infectivity of zoospores in the soil under laboratory conditions. For how long the zoospores remain motile in the soil is one of the many unanswered questions in the zoospore biology. From the present study it is seen that, the sporangia can germinate in the soil and liberate zoospores. The zoospores can move against gravity, remain viable and infective for 5 hrs in the soil. Survival of zoospores in the soil indicated that, they may serve as a potential secondary source of inoculum through soil under field conditions.

Ridge gourd pollen has a stimulatory effect on the germination ofPseudoperonospora cubensis. The rate and percentage germination of zoospores increased in the presence of pollen leachates. Spraying of leaves with a mixture of pollen and sporangial suspension enhanced the development of lesions. Early germination of zoospores in the presence of pollen proved advantageous for infection as it provided a prolonged favourable infection period. The results are discussed in relation to the epiphytoties of the disease during flowering period.

Oospores ofPeronospora parasitica are found to be dependent on environmental factors such as temperature, light, pH of the medium and age of oospores. Optimum temperature of 23°C is required for their germination. Drying and chilling had no marked effect on oospore germination. At pH 7·5, 42% germination was recorded while at pH 4·5 only 1% of oospores germinated. Oospore germination increased with increase in their age.

InPeronospora parasitica the inoculum load is found in the form of oospores in the leaf and seed tissues of radish. Out of 400 seeds tested, 10% showed the presence of oospores in the pericarp and 0·1% in the embryo. The 2,3,5-triphenyltetrazolium chloride test is a quick method of determining the viability of the oospores. Viability of oospores based on infection capacity after storage, though a long process, is effective and reliable. Results ofin vitro andin vivo experiments show that the oospores need natural weathering, under field conditions, for a period of one year for maximum infection in radish and those stored for two years under the same conditions, has an adverse effect on their infection capacity. Infection capacity was higher among oospores exposed to weathering than those retained in laboratory.

The alkali maceration technique was used to detect the seedborne nature ofPeronospora parasitica inRaphanus sativus. Four cultivars ‘Japanese white’, ‘Arka nishant’ ‘Pusa desi’ and ‘Pusa reshmi’ were used to confirm the presence of pathogen in the seed. The percentage of embryonal infection in the cultivars were 12·5, 0·5, 0·25 and 0·1 respectively. The percentage of seedling infection is directly correlated to the percentage of embryo infection. The possibility of using this technique in quarantine screening is discussed.

The entry ofPeronospora parasitica conidia through stigma, ovary wall and its establishment in the ovary is clearly demonstrated. The pathogen also enters directly through the inflorescence axis of the mother plant. The infection through stigma and ovary wall results in embryonal infection. The infected seeds transmit the downy mildew disease and a direct correlation is noticed between embryo infection and seed transmission of the pathogen.