Mycelial Interaction Zones Among Single Ascospore Isolates of Monilinia Fructicola

Abstract

Thind and Keitt (5) reported that when colonies of Monilinia fructicola (Wint.) Honey [Sclerotinia fructicola (Wint.) Rehm.] originating from ascospores of the same ascus grew together on potato-dextrose agar, light to dark, single or double lines formed at the margins where colonies met. No lines were reported between colonies of single ascospore isolates arising from the mitotic division during ascospore development or between subcultures of the same single ascospore isolate. Thind and Keitt (5) paired mycelium of 20 distinct single ascospore isolates from five asci and found that lines formed between all colonies. If mycelial interaction zones (barrages) occur between colonies of any pair of ascospore isolates and all other isolates, then we can use this interaction to tentatively identify isolates released in an orchard and follow their progress. If Thind's and Keitt's finding can be extrapolated to all isolates of M. fructicola, any isolate from the orchard not forming an interaction with subcultures of the ascospore pair may be a clone of the released isolate. Our study was conducted to determine if mycelial interactions consistently occur among single ascospore isolates of M. fructicola. A preliminary laboratory study using single conidial isolates of M. fructicola was conducted to determine the media best suited for the development of zones of interaction. The media tested were freshly prepared potato-dextrose agar (FPDA), FPDA acidified with lactic acid (AFPDA), Difco PDA, Difco PDA acidified with lactic acid (Difco APDA), Difco Czapek's agar, Difco malt agar, Difco corn meal agar, V-8 juice agar, and oatmeal agar (OMA). Ten single-spore conidial isolates of M. fructicola were obtained from diseased peach fruit from an orchard in Lockeford, California. The ten isolates were paired on each media by placing 4 mm disks of one isolate on adjacent corners of a 3 cm x 3 cm square and two disks of the other isolate on the remaining corners. The plates were incubated for 10 da at 20 + 2 C in a dark cabinet, under 24 h fluorescent light, and in intermittent light on a laboratory bench. Mycelial interaction zones formed on OMA under all three light regimes where colonies of different isolates met, but not among colonies of the same isolates. Interaction zones on OMA were darker on plates incubated in the dark or under intermittent light than plates incubated under 24 h fluorescent light. Interactions were inconsistent on FPDA, AFPDA, Difco PDA and Difco APDA in the dark or in intermittent light and on Difco PDA and Difco APDA under 24 h fluorescent light. On FPDA and AFPDA under 24 h fluorescent light, interaction zones were consistent but the lines were faint. No obvious interaction zones occurred on other media. Seven ascocarps were collected from an orchard in Lockeford (San Joaquin