. Bulletin. Natural history; Natural history. September 1985 125 Years of Biological Research 221. Fig. 1. - Biotic stresses as primary factors affecting Illi- nois trees. (A) Ver- ticillium wilt on green ash. (B) Nee- tria canker on thorn- less honey locust. (C)Sporulating cedar- apple rust galls on red cedar. (D) An- thracnose lesions on black walnut. Consequently, disease outbreaks and epidemics are influenced by environ- mental conditions of light, temperature, and humidity (Colhoun 1973). If a sus- ceptible host and a virulent pathogen are present and environmental condi- tions favor infe
. Bulletin. Natural history; Natural history. September 1985 125 Years of Biological Research 221. Fig. 1. - Biotic stresses as primary factors affecting Illi- nois trees. (A) Ver- ticillium wilt on green ash. (B) Nee- tria canker on thorn- less honey locust. (C)Sporulating cedar- apple rust galls on red cedar. (D) An- thracnose lesions on black walnut. Consequently, disease outbreaks and epidemics are influenced by environ- mental conditions of light, temperature, and humidity (Colhoun 1973). If a sus- ceptible host and a virulent pathogen are present and environmental condi- tions favor infection, disease damage will appear. Disease outbreaks reach epidemic proportions when pathogen inoculum is abundant and large num- bers of susceptible hosts are available, as is the case with street and parkway plantings in which species diversity is lacking. Some spore-bearing fungi and certain bacteria are spread from tree to tree by wind and rain, and others are transmitted by animals, primarily in- sects. Where pathogens and host trees have coexisted for many years, trees have usually developed resistance to disease through mutation and natural selection. Pathogens introduced into a population of trees that have not developed resistance can cause devas- tating disease epidemics. In 1934, the Dutch elm disease fungus came into this country from Europe. The Ameri- can elm, which was widely planted as a parkway tree throughout North America and was also a common woodland species, had virtually no resistance to the fungus. As popula- tions of the European elm bark beetle vector built up, an epidemic of Dutch elm disease began in the east and is still spreading toward the west coast. As already mentioned, the chestnut blight fungus, which spread rapidly by wind- and rain-carried spores, killed. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfec
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