Germination and invasion by ascospores and pycnidiospores of Leptosphaeria maculans on spring-type Brassica napus canola varieties with varying susceptibility to blackleg

The infection processes of ascospores andpycnidiospores of Leptosphaeria maculans were studied oncotyledons of six cultivars of spring-type Brassica napus:one with resistance controlled by a single dominant gene(cv. Surpass 400), three with polygenic resistance (cvs.Dunkeld, Grouse, and Outback), and two susceptible cultivars(Westar and Q2). On all cultivars, ascospore germination,penetration, and development of symptoms oncotyledons were much earlier than that with pycnidiospores.At 2 h after inoculation ascospores began to germinate,by 4h about 50% had germinated, and by 6–8h85%–90% had germinated. In contrast, pycnidiosporesbegan to germinate 1 day after inoculation (dai) andreached only 50% germination by 3 dai. Ascospores begangerminating from terminal cells and then later from theinterstitial cells. Pycnidiospores germinated predominantlyfrom one end and sometimes from both ends. Germ tubesfrom ascospores penetrated stomata as early as 4h afterinoculation, whereas those from pycnidiospores penetratedat 2 dai. Symptom development with ascospores was 2 daysearlier than that with pycnidiospores. Symptoms on Surpass400 were evident as early as 3–5 dai with ascospores and 5–7 dai with pycnidiospores. However, on other cultivars,symptoms were not evident until 10 dai with ascospores and12 dai with pycnidiospores. This report is the first on differencesin the infection processes by the two spore types.Ascospore and pycnidiospore attachment, germination, andpenetration did not differ between resistant and susceptible cultivars, but there were major differences after penetration.Under high humidity, 80%–90% of stomata of susceptibleWestar and Q2 had aerial hyphae emerging fromstomatal pores. However, fewer stomata (5%–10%) hadaerial hyphae on Surpass 400 by 10 dai with ascospores and12 dai with pycnidiospores, but even these were usuallypoorly developed. Host differences in spring-type B. napusin relation to production of aerial hyphae have not previouslybeen reported. In Surpass 400, rapid necrosis of guardcells occurred within a few hours of penetration by eithertype of spore, and subsequently one or a few cells immediatelyadjacent to the penetration site died. This necrosisthen spread to the cells around the penetration site to forma hypersensitive response (in the form of a small, darklesion) to both ascospores and pycnidiospores. This is thefirst detailed report on interactions between spring-type B.napus and L. maculans in relation to single dominant genebasedresistance. Neither the cultivars with polygenic resistancenor the susceptible cultivars had such a response.