Conidial sporulation of Sclerospora graminicola has been observed in the day time occurring naturally in the field and under artificial conditions in the laboratory. The period required for the development of mature conidia lies between 4 hours 35 minutes and 111/2 hours. The conditions which seem necessary for the production of conidia, whether during the day or night, are: a completely saturated atmosphere, turgid host leaves, a slight moisture film on the surface of the leaves and a temperature ranging between 8° and 27 °C.
When flooded into a drop of water immediately after being discharged, conidia were found to germinate after 60 minutes. The best germination was obtained at 15°C., but the optimum was not definitely determined.
Normal conidia measure 14-23 x 11-17µ. Sometimes larger conidia are produced (43 x 18.6µ) .
The average length of conidiophores of Sclerospora graminicola was found to be 267.8µ while the individuals measured ranged from 214.5 to 375.3µ, a variation of 160.8µ.
Spores were found to be forcefully discharged from the conidiophores thru a distance of 2.5 mm. vertically and 1.89 mm. horizontally. Setaria viridis, S. italica, Zea mays (May\u27s Golden popcorn) and Euchlaena mexicana were infected with Sclerospora graminicola when exposed to the conidia of the mildew.
Oospores of Sclerospora graminicola were found to overwinter naturally in field soil under Iowa conditions. In one test, oospores which overwintered outdoors gave nearly twice as much infection on Setaria viridis and Zea Mays (Japanese Hulless popcorn) as did the oospores which were kept in the laboratory, Plants from five genera of Gramineae were found to be susceptible hosts to Sclerospora graminicola. These are: Euchlaena, Setaria, Holcus, Saccharum and Zea. Setaria viridis was found to be the most susceptible of all hosts and popcorn more susceptible than sweet corn and dent corn.
Six days were found to be the usual period of incubation between the time oospores were placed on the seeds and that when conidial fruiting appeared on the leaves.
Infection by oospores was obtained from the time the testa was broken until the emergence of the plumule above ground. Relative susceptibility of seedlings decreases with age. The processes connected with infection are more greatly favored by temperatures of 15° to 16°C. than by temperatures of 24° to 30°C.
The germinating oospore is evidently unable to penetrate older leaf tissue.
The viability of oospores was little affected by soaking in 2 percent copper sulfate solutions for 10 minutes, while similar treatment in 1 percent formaldehyde for 5 minutes proved fatal. The killing action of mercuric chloride 1-1000 was not so great as that of formaldehyde.
Freshly collected oospores which were held in a dry condition at 77°C. for 1 hour later gave 52 percent infection on Setaria viridis, while wet spores lost their viability to a marked degree when held at 50°C. for a similar period.
Sclerospora graminicola was studied in the field during the summers of 1925, 1926 and 1927. Infection was obtained on corn and teosinte planted in plots which had been artificially infested with oospores. Spontaneous conidial sporulation was found to be comparatively rare on corn in the field altho it was observed in 1926 and 1927 on young seedlings during periods of high humidity and cool temperatures. Infected plants were either killed outright or became stunted and unproductive. A few plants apparently outgrew the attack.
In Iowa, Sclerospora graminicola has been observed only twice occurring naturally on corn in the field.
Oospores of Sclerospora graminicola which had been held 30 months under dry conditions in the laboratory were found to be viable.
Presoaking of oospores does not seem to affect the percentage of infection.
Soil is not necessary as a medium for the germination of oospores
