ABSTRACT
ABSTRACT Twigs with constriction cankers were pruned from a Prunus persica 'Jerseyglo' orchard and placed in incubators under high humidity (>95%) at constant temperatures of -6, 1, 10, 17, 24, 31, 38, and 45 degrees C. Cankers were removed for observation after 2, 6, 24, 48, 72, and 96 h incubation. Sporulation was expressed as the percentage of pycnidia producing cirri and the number of conidia per pycnidium or canker. The experiment was first performed during 1998 to 1999 and then repeated in 1999 to 2000 and 2000 to 2001. Sporulation was modeled by fitting the Richards function to each dependent variable with duration of high relative humidity as the independent variable. The asymptote and rate parameters of the Richards model were expressed as Gaussian and quadratic functions of temperature, respectively. Models created from the first two experimental repetitions were validated by statistical comparison to those created independently from the third repetition. When models were fit to the pooled data, temperature and moisture described 69 to 80% of the variation in sporulation. The models specified a temperature ranging from 0 to 37 degrees C (optimum temperatures for cirri formation and conidia production range from 19 to 20 and 22 to 23 degrees C, respectively), and the majority of sporulation occurred between 16 and 48 h from initiation of the high humidity period. These results show that the environmental criteria for sporulation coincide with those that prolong the susceptibility of infection courts during fall and spring.
ABSTRACT
ABSTRACT The seasonal sporulation ability of Phomopsis amygdali was evaluated over a 2-year period by sampling twig cankers from commercial 'Encore' and 'Jerseyglo' peach orchards. Canker size, pycnidia per canker, percent pycnidia forming cirri, and spore production were evaluated once each month from January 1997 through December 1998. Average canker size and number of pycnidia per canker, which followed a sinusoidal pattern of change, were at lowest values in spring and at maximum values in fall. In 1997, the ability of pycnidia to sporulate (produce cirri) increased significantly from a minimum during winter to a maximum during summer. In contrast, pycnidia sporulation in 1998 increased slowly throughout the year with a maximum in the fall. Inoculum potential, measured as number of spores per canker, peaked between June and August and was lowest in winter. Although pycnidia numbers in 1997 were about 50% of that observed in 1998, approximately twice as many conidia per canker were produced in 1997; consequently, the number of conidia produced per canker was equivalent in both years. These results indicated that P. amygdali acts as an r strategist by maintaining abundant pycnidia and sporulation capability throughout much of the year. Therefore, infection is most likely not limited by the availability of inoculum.
