Leptotrochila medicaginis: Moisture Requirements for Ascospore Discharge, Germination, and Plant Infection. G. Semeniuk, Professor Emeritus, Department of Plant Science, South Dakota State University, Brookings 57007. . Plant Dis. 77:37-42. Accepted for publication 21 August 1992. Copyright 1993 The American Phytopathological Society. DOI: 10.1094/PD-77-0037.

In humidity-controlled atmospheres above water or different concentrations of aqueous NaCl solutions at 20–22 C, mature apothecia on dry, infected alfalfa leaves (Medicago sativa) rehydrated and discharged ascospores of Leptotrochila medicaginis more abundantly near 99.5 and 100% RH and less abundantly at progressively lower RH values; RH limits for discharge were 98.0% at 14.4 and 15.5 C and 97.5% at 20–22 and 26-28 C. On glass in a moist chamber (~100% RH) and on fresh water agar (WA) at 20–25 C, discharged ascospores swelled and formed short germ tubes within 4 hr, which lengthened to spore width (approximately 5 μm) within 6 hr. Ascospores on fresh potato-dextrose agar completed these two processes within 4 hr. They also completed these two processes on WA fortified with glucose, glycerol, and sucrose as humectants within 4 hr at water potentials ranging from –40.6 to –30.7 bar and within 12–24 hr at water potentials ranging from –70.8 to –63.6 bar, and on WA fortified with NaCl and KCl as humectants within 24 hr at water potentials of –49.2 and –63.6 bar, respectively. Ascospores on alfalfa stem cuttings held in a moist chamber at 20–25 C swelled within 4 hr, and their germ tubes infected leaves within 8 hr. At a constant 17 C, the minimum RH for leaf infection within 24 hr was 95%.

Keyword(s): yellow leafblotch.