are required to reduce the leaf surface significantly and affect yield. Dry weather 

 retards the spread of this disease. 



The symptoms may appear in bands as infection occurs by swimming spores (zoospores) 

 that land in whorls filled with water from heavy dews or rain. Corn may lodge if 

 considerable leaf damage and premature death of the plants occur. Sporangia (fruit- 

 ing bodies) survive the winter in old infected plants and in the soil. 



Primary control measures for Physoderma include plow down, crop rotation, and use of 

 resistant varieties. 



Sudden appearance of lesions only on the top leaves of T corn would indicate wind- 

 borne movement of the spores of Helminthosporium maydis race T. 



BLIGHT DAMAGE MAY BE LESS THAN IN 1970 



Despite heavy rains during the past two weeks, the build-up of southern corn leaf 

 blight infection has not been as great as feared earlier. At present infection lev- 

 els, even if conditions for blight remain ideal until harvest, the amount of damage 

 likely to occur from the disease will be less than the actual damage in 1970. Here's 

 why: 



1. Recent heavy rains washed blight-causing spores out of the air and off the plants. 

 Light, gentle showers or heavy morning dews followed by periods of slow drying- - 

 overcast skies- -are more favorable for development of blight infections. 



2. Hot, dry conditions before the recent rains did not favor the formation of addi- 

 tional spores. Also, spores have not been blown up from southern states- -the 

 primary source of the blight -causing fungus last year. The spread north did not 

 occur because a major portion of the corn crop planted in the southern states is 

 in resistant seed. 



5. A recent U.S. Department of Agriculture report indicates that about 50 percent of 

 the 1971 Illinois corn crop is normal (N) cytoplasm corn. The estimate includes 

 fields planted to straight N corn and the N corn contained in blends. In 1970 

 only 15 percent of the Illinois corn crop was in N cytoplasm. 



4. Blight development seems slightly ahead of last year. But the Illinois corn crop 

 is also ahead of last year in stage of growth. 



In 1970 about 50 percent of the Illinois corn acreage was planted in late May and June. 

 In contrast only about 5 percent of the 1971 crop was planted after June 1. 



Infections during the early- tassel to early-dent stage are most likely to cause ser- 

 ious damage. Infections after the dent stage cause less damage. 



As a rule of thumb, denting usually begins 56 days after corn has silked. Most of 

 the Illinois corn crop is now in the silk stage, so the next 56 days will tell the 

 story. 



Best advice is to "walk" corn fields and tag plants to check blight development. If 

 10 or more lesions appear on each of the upper leaves of susceptible corn- -and the 

 crop has not yet reached the dent stage- -use of a protective fungicide may be war- 

 ranted. Be sure to check stalks and ears as well as leaves for signs of blight in- 

 fection. 



