Influence of Temperature and Moisture on Germination of Ascospores and Conidia of Botryosphaeria obtusa

Arauz, L. F., and Sutton, T. B. 1989. Influence of temperature and moisture on germination of ascospores and conidia of Botryosphaeria obtusa. Phytopathology 79:667-674. Maximum germination of conidia of Botryosphaeria obtusa occurred requirements for germination. Requirements for ascospore germination in free water and declined as relative humidity (RH) was reduced from were similar to those for conidia. Germ tube lengths of ascospores and 100 to 92%; no germination was observed at 88.5% RH. Germination conidia were maximum in free water and decreased with decreasing relative in free water reached 80% in 4 hr at 16-32 C and in 12 hr at 12 C humidity. After 12 hr in free water, ascospore germ tubes reached a but was only 23 and 0% at 8 and 4 C, respectively, after 12 hr. Higher maximum mean length of 0.78 mm at 28 C, whereas maximum conidial temperatures were required for germination at 95 and 92% RH (16 and germ tube length was 0.82 mm at 24 C for isolate 087 and 0.99 mm 28 C, respectively) than at 98, 99, and 100% RH (12 C). Conidial at 28 C for isolate 049. Germ tube length of both types of spores declined germination at 92% RH was observed only after 12 hr at 28 C. The at 32 C. two isolates tested differed in temperature and relative humidity The fungus Botryosphaeria obtusa (Schw.) Shoemaker Infection of apple by B. obtusa is favored by free water or (synonym Physalospora obtusa (Schw.) Cooke) is found high relative humidity (RH) (6). The precise influence of relative throughout most tropical and warm temperate regions in the world humidity on spore germination of B. obtusa has not been as a pathogen of many crops (4,5). The greatest economic losses determined, although, in general, relative humidity plays a major due to this fungus occur on apple (Malus X domestica Bork.), role in germination of most fungal spores (13). on which it causes fruit black rot, frogeye leafspot, and a limb Foster (6), in a study on the influence of temperature on conidial canker (10). Fruit losses as high as 60% have been reported (15). germination of B. obtusa, found that approximately 90% of the Conidia are the main propagules of B. obtusa in apple orchards conidia germinated at temperatures between 12 and 32 C. After throughout the growing season in North Carolina although a 24-hr incubation in free water, optimal temperature for germ ascospores can be important early in the season (14). Conidia tube elongation was 24 C, although the average germ tube length have been implicated as the main spore type involved in primary was similar from 20 to 32 C. He did not study the effect of dispersal of B. obtusa in Georgia (3). relative humidity on germination of conidia nor did he work with ascospores. ___________________________________________ Knowledge of the factors that influence the germination of © 1989 The American Phytopathological Society conidia and ascospores will result in a better understanding of Vol. 79, No. 6, 1989 667 factors favoring infection and may lead to the development of collected in western (isolate 049) and central (isolate 087) North more efficient management programs for the diseases caused by Carolina, were grown under continuous light on cellulose films B. obtusa. The objective of this study, therefore, was to determine (Flexel Sales, Inc., Covington, IN) placed on top of oatmealthe influence of temperature, relative humidity, and their agar medium (Difco Laboratories, Detroit, MI). Conidia were interaction on the germination of conidia and ascospores of B. obtained from cultures 12-14 days old by scraping off the obtusa. nonsporulating mycelium from the cellulose films and blending the remaining mycelium, pycnidia, and cellulose films for 30 sec MATERIALS AND METHODS in sterile distilled water in a commercial blender. The resulting suspension was strained through three layers of cheesecloth to Conidia. Two isolates of B. obtusa, obtained from apple fruit separate the conidia. The spore suspension was standardized to 120,000 conidia/ml to provide a minimum of 100 conidia/2-gl drop of suspension. NaCl-amended agar 9-cm Petri plate The agar dish isopiestic equilibration technique, described by Harris et al (8) and modified by Alderman and Beute (1), was used to control the relative humidity. One drop of the conidial suspension of either isolate 049 or 087 was placed on each of four microscope cover glasses by means of a micropipetter (Pipetman P-20, Rainin Instrument Co., Inc., Emeryville, CA) and air dried. The four cover glasses, corresponding to four assessment times, were placed in a petri dish containing 44 ml of water agar amended with sodium chloride as shown in Figure 1. Each dish was sealed with Parafilm (American Can Company, cover glass with spore sample Greenwich, CT). The relative humidity in this sealed atmosphere Fig. 1. Relative humidity chamber showing position of cover glass with is related to the NaCl molality according to the values given spore sample. by Lang (11). The relative humidities tested were 100, 99, 98, 100 o00 o 90 I s olate ;C..... ."' o 90go 2 80 087 ,.4, 8 //0 . 70 ... C49. 7060 60 CM 50 cu,7 50 40 -/40 / C:C 30 a 30 20 ; • 20 c 10 tA EL 000r 0 8 16 24 320 8 16 242 100 i 00 090 ,. 0 90 280 " c 70 An .e00 E 60 : I 7