Ue 



1958 



Carter: Apimjud Hotanv and Plant Pathology 



151 



The plant disease survey not only in- 

 dicates the annual losses caused by plant 

 diseases but reveals diseases new in the 

 state and the sudden and widespread 

 damage caused by any disease that has 

 caused only minor damage in preceding 

 years. Downy mildew of alfalfa appeared 

 generally in the state and was abundant 

 in the extreme north in 1924. This disease 

 had not been seen in Illinois previous to 

 that year. A new leaf spot of cowpea was 

 discovereci in Clinton County in 1927. In 

 an article by Stout (1930), 16 new fungi 

 found on corn in Illinois were described. 

 Downy mildew of soybean, first reported 

 in Illinois in 1929, caused considerable 

 damage in 1935, when it was found in 

 12 counties. Brown stem rot of soybean, 

 first recognized in the state in 1944, sud- 

 denly became widespread and destructive 

 in 1948. This outbreak of the disease fol- 

 lowed a fortnight of low temperatures, 

 which ended on August 10. Septoria leaf 

 spot of broom corn was discovered in Illi- 

 nois in 1949 and was very destructive in 

 several fields west of Galton in Douglas 

 County. 



Diseases recorded for the first time in 

 Illinois in recent years include ergot on 

 timothy, bacterial blister spot on apple, 

 charcoal rot on pepper, and downy mil- 

 dew on wheat in 1952; basal glume rot 

 on barley, anthracnose on sweet clover, 

 and rosette on cherry in 1954; Ascochyta 

 leaf spot on rhubarb and bacterial leaf spot 

 on mulberry in 1955; powdery mildew on 

 apple, ergot on oats, Helminthosporium 

 leaf spot on red top, and Gloeosporium 

 leaf spot on currant in 1956; and Phy- 

 tophthora root rot on alfalfa, Phytoph- 

 thora stem rot on lily, Cercospora leaf 

 spot on Deutzia, Abelia, ornamental 

 gooseberry, and wafer ash, downy mil- 

 dew on cucumber, squash, and water- 

 melon, rust on apricot, anthracnose on 

 iris, powdery mildew on pecan and frag- 

 rant sumac, Badhamia slime mold on 

 timothy, Herptobasidium scorch on bush 

 honeysuckle, and Phyllachora tar spot on 

 lespedeza in 1957. 



In the plant disease survey, not only 

 are the various kinds of crops examined 

 but many plants in many fields of the 

 same crop are examined each summer. 

 For instance, in 1949, data on prevalence 

 and severitv of wheat diseases were ob- 



tained by examination of plants in 42 

 wheat fields that totaled 1,033 acres and 

 that were located in 38 widely scattered 

 counties of the state. 



Another phase of the plant disease sur- 

 vey is that of forecasting the anticipated 

 occurrence and seriousness of plant dis- 

 eases. This forecasting has been notably 

 efifective for Stewart's disease of corn. 

 The bacterium that causes Stewart's dis- 

 ease overwinters chiefly in the body of the 

 adult corn flea beetle (Chaetocnema puli- 

 caria) . The mortality rate of the flea bee- 

 tle is affected by weather conditions dur- 

 ing hibernation. 



Although forecasting the early season 

 or wilt stage of Stewart's disease had pre- 

 viously been worked out bv others, fore- 

 casting the late season or the leaf blight 

 stage was worked out by G. H. Boewe. 

 Making use of data accumulated in the 

 5-year period 1944-1948, Boewe found 

 that a winter temperature index rather 

 accurately forecast the late season develop- 

 ment of Stewart's disease. The index for 

 any growing season was based on the sum 

 of the mean temperatures of the previous 

 winter months of December, January, 

 and February. While early season epi- 

 demics do not develop unless the index is 

 90 or above, light to moderate late sea- 

 son epidemics develop when the indexes 

 are between 80 and 85, and moderate to 

 heavy late season epidemics when the in- 

 dexes are above 85. No disease or only a 

 trace of disease develops when the in- 

 dexes are below 80. Forecasting of the 

 severity of disease each year has been 

 quite accurate. 



The appearance and spread of new dis- 

 eases on crops in Illinois often are re- 

 corded first as a result of the annual sur- 

 vey made for plant diseases. Aid to farm- 

 ers in combating these diseases is made 

 through warnings and through publica- 

 tions such as Diseases of Small Grain 

 Crops in Illinois (Boewe 1939). 



Fruit Diseases. — Of the many dis- 

 eases that affected fruit trees in the state 

 each year during the early years of the 

 plant disease survey, the most common 

 and destructive were scab, shothole, 

 brown rot, and leaf curl of peach ; fire- 

 blight, frogeye, and blotch of apple ; fire- 

 blight, leaf blight, and leaf spot of pear; 

 and shothole and leaf spot of cherry. 



