-5- 

 PLANT DISEASES 



DEVELOPMENT AHEAD OF THE BLIGHT 



The growth stage of the crop- -not the date on the calendar- -makes the difference when 

 comparing the development of southern corn leaf blight this year with last season. Mich 

 of the Illinois corn crop was planted early this year. The relatively high temperatures 

 early in the season got the crop off to a good start. As a result, the 1971 crop is 

 closer to maturity than the 1970 crop was at this time a year ago. 



As corn plants approach maturity, there is less damage to the plant and less potential 

 yield reduction. The plant pathologists generally agree that blight infections occurring 

 after the corn reaches the dent stage have a relatively minor effect on the crop. 



Nearly all of the Illinois corn crop has now reached the silk stage. Agronomists say 

 that plants usually require 12 days to advance from the silk to the blister stage; 24 

 days, to the dough stage; and 36 days, to the beginning of dent stage. Another 12 days 

 are required to complete denting. At 60 days after silking, corn is generally physio- 

 logically mature- -at 30 to 35 percent moisture. 



Although the overall effects of the blight may be less severe than last year, this does 

 not mean that individual corn producers will not suffer greater losses than in 1970. In 

 some areas of the state- -particularly in a belt through south-central Illinois where 

 conditions for blight development have been favorable- -the potential for major blight 

 damage still exists. 



If infection is already present and if susceptible corn was planted, watch such fields 

 closely. Tag several plants in the field and check every day or two for signs of blight 

 development. 



Some situations may warrant spraying to protect the crop from further infection. The 

 decision to spray must be based on conditions in individual fields. First weigh the 

 economic factors to decide whether spraying will pay. If spraying is chosen, consider 

 the timing, materials, rates, and application methods that will provide the most pro- 

 tection. Both aerial and ground application equipment should give equally effective 

 results when correctly adjusted and properly operated. County Extension advisers have 

 guidelines for decision-making and recommendations for rates, timing, and applications. 

 Also, check the June 25 issue of this bulletin. 



As a general rule, begin spraying when about 10 lesions appear on the upper leaves of 

 25 to 50 percent of the plants in a susceptible (T or blend) corn field. Repeat the 

 spray applications every 7 to 10 days, depending on the weather conditions. Corn that 

 has reached the dough stage before this level of infection appears will probably per- 

 form satisfactorily even if sprays are not applied. If more than 40 lesions are present 

 on each of the upper leaves of susceptible plants, it is probably too late to spray. 



READ THE LABEL AND FOLLOW ALL PRECAUTIONS 



This weekly report was prepared as follows: 



INSECTS: H.B. Petty, Steve Moore, Roscoe Rand ell, Don Kuhlman, and Tim Cooley , College 

 of Agriculture , University of Illinois at Urbana-Champaign , and the Illinois Natural 

 History Survey. 



WEEDS: Ellery Knake and Marshal McG lamer y , Department of Agronomy. 



PLANT DISEASES: M.C. Shurtleff and Ed Burns, Department of Plant Pathology. 



AG COMMUNICATIONS: Ray Wood is 



The information for this report was gathered by these people, staff members, county 

 Extension advisers, and others, in cooperation with the USDA Agricultural Research 

 Service, Plant Pest Control Branch. 



