The pathogenicity and seasonal development of Gymnosporangium in Iowa
Seven species of the genus Gymnosporangium occur in Iowa, but only G. juniperi-virginianae, causing cedar-apple rust, and G. globosum, causing hawthorn rust, are common. The latter is of little economic importance.
The epidemic of cedar-apple rust in 1928 caused an estimated loss of $200,000 in Iowa.
Six to seven hours were required to produce abundant sporulation from air dry galls of G. juniperi-virginianae regardless of whether the period of soaking was 30 or 180 minutes.
Newly-matured aecidiospores of G. juniperi-virginianae germinated to the extent of 54 percent in July. A lot was divided and the percentage of germination in the portion refrigerated at 5 to 13°C. increased rapidly for at least 22 days, while that in the portion held at room temperature for the same interval dropped to zero.
At Shenandoah the interval between infection and first opening of aecidia of cedar-apple rust on Bechtel's flowering crab was about 72 days. That for hawthorn rust on Crataegus mollis was about 81 days.
Seedlings of red cedar were successfully inoculated using aecidiospores of G. juniperi-virginianae. Large differences in the number of galls on trees which had received similar treatments suggested a variation in disease resistance.
The only appreciable germination of aecidiospores of G. globosum was obtained in November from diseased hawthorn leaves which had fallen to the ground and others which were refrigerated 41 days at 5°C.
Differences in the degree of maturity reached by the cedar rusts are utilized in defining the relative susceptibility of their aecidial hosts.
The percentage of leaf area covered by lesions of cedar-apple rust is recorded for 138 varieties of apples and 28 varieties of crabapples which were growing in nursery rows at Shenandoah during the years, 1928 to 1930. Aecidiospores were matured on only 30 percent of these hosts.
The percentage of infected apple leaves seemed to vary roughly as the percentage. of diseased leaf area, but leafiness was not related to the amount of rust infection. These lines of evidence suggest that the period of susceptibility in leaves of susceptible varieties may be longer than that of resistant ones.
Artificial inoculations show that Tolman and York Imperial varieties of apple are very susceptible to the strain of G. juniperi-virginianae from Morgantown, W. Va., but are resistant to other strains from Ames and Shenandoah, Iowa; Manhattan, Kan., and Cochrane, ,Vis. The results indicate the presence of physiologic specialization within the species. There is little evidence that the susceptibility of trees of one variety of apple may vary in different sections of the country.
None of the observed varieties of apple or crabapple seem to be susceptible to G. globosum, in Iowa.