Oct. 1. 1920 Fusarium-B light (Scab) of Wheat and Other Cereals 1 7 



METHOD OF PERFECT STAGE DEVELOPMENT 



In nature. — A limited study of the field conditions under which the 

 perfect stages of some Fusarium species which parasitize the cereal crops 

 and numerous grasses are formed showed that those conditions are as 

 follows : 



(1) Successful parasitism of the fungus on some host. The perithecia 

 are formed usually and preferably on those dead parts of the host which 

 have been parasitized. 



(2) Successful conidia production. Conidia production on the in- 

 fected substratum, root, stems, or heads always precedes the formation 

 of perithecia, since the latter are formed more readily on the crust or 

 plectenchymatic layer formed by the conidia- bearing hyphae and the 

 germinated masses of conidia themselves. 



(3) Presence of moisture. No perithecia will ever be formed in the 

 absence of sufficient moisture, and their formation will be delayed until 

 moisture is sufficient. 



(4) Suitable temperature also must play some role in the formation 

 of the perithecia. Formation of perithecia took place only during the 

 summer when the temperature was highest. Efforts to develop the per- 

 ithecia from infected material during October and November gave nega- 

 tive results. 



When the foregoing conditions were established as factors in the for- 

 mation of perithecia, the following method of producing them was 

 worked out and has yielded good results. The infected parts of the various 

 cereals, including corn, such as stems, nodes, sheaths, heads, and ears, 

 were gathered from the field and laid on the ground during July and 

 August, 1 91 8, then covered with a wire screen, moistened thoroughly, and 

 covered with some dry grass and leaves to protect them from drying out. 

 During the first and second weeks, masses of conidia were formed over the 

 entire infected area of the various parts. Soon this extended even over the 

 uninfected area. Before long all conidia germinated and no others were 

 formed. During the third week the perithecia began to be formed. In 

 three or four more weeks numerous perithecia were formed, most of them 

 with matured ascospores. 



The following is a record of one of the experiments for perfect-stage 

 development : 



June 28, 1918. Rye heads infected with Gibberella saubinetii were placed under 

 screens so as to be exposed to the action of the weather. They were sprayed 

 thoroughly with water and covered with dry grass to protect them from drying out. 



July 16, 1918. First perithecia beginning to appear. 



August 2, 1918. Numerous perithecia formed, but asci not yet fully developed. 



August 21, 1918. All perithecia have ripe ascospores. Heads taken to the laboratory 

 for study. 



In laboratory. — Infected wheat kernels, when placed in a pot filled 

 with fine sand and only slightly covered with sand and kept moist at 

 187931°— 20 2 



