Culture characteristics and variation of Cronartium flaccidum isolates

Cultures of Cronartium flaccidum were established from aeciospores collected from Pinus halepensis and Pinus laricio growing at four Italian sites: one southern and one northern (north-central) site for each host species. The two southern and the two northern sites were also geographically separate. Colonies were incubated for 7 months and compared for growth rates, macroscopic and microscopic features of the colonies, presence or absence of sporulation, and spore type. Germ tubes exhibited negative geotropism and were light sensitive. Colonies from southern isolates generally remained white and fast growing throughout, whereas colonies from northern isolates tended to turn orange and become slow growing. The faster growing white colonies could be maintained in the same Petri dishes without undergoing morphological change for the entire incubation period, but the slow-growing orange colonies had to be transferred to fresh medium every month or they became covered with short, hyaline hyphae. Immature aeciospores, typical mature aeciospores, and teliospore-like bodies formed in culture. Spore ontogeny, capacity to be successfully subcultured, growth rate, and other evidence indicated the southern and north-central Italian isolates were distinct. The host species was not a factor in culture variation. The importance of the axenic technique in relation to rust disease research is discussed. Keywords: rust, in vitro growth, cultural variation, sporulation.