HOST SPECIALIZATION IN MICRODOCHIUM NIVALE ON CEREALS
BM COOKE and H DIAMOND
1 University College Dublin, Belfield, Dublin 4, Ireland
Background and objectives
Microdochium nivale is an important pathogen of cereals . Prior to 1980, the fungus was classified as Fusarium nivale, despite the absence of a pedicillate basal cell in conidia and the production of a teleomorph not assigned to the Hypocreacae. Isolates of M. nivale can be differentiated into two with M. nivale var. majus having larger conidia than M. nivale var. nivale. Observations suggest that var. majus may have a stronger pathogenicity to wheat than var. nivale, so the distinction may have practical importance. PCR-based technologies have enabled diagnosis of M. nivale sub-groups  . Work has shown RAPD profiles distinguish a subgroup of M. nivale that relates to M. nivale var. majus, on the basis of conidial morphology. However, there is a greater degree of variation within the var. nivale subgroup. Isozyme analyses of M. nivale has shown banding patterns constituted by subgroups leading to intermediate characteristics not easily distinguished by morphological taxonomy, illustrating diversity within the heterogeneous variety nivale. In current work, an analysis of M. nivale isolates from seed of wheat, oats and barley from different locations in Ireland during 1994 and 1996 was undertaken using RAPDs to determine the relative incidence of each variety on each host. Conidial morphology was also measured. Host specialization was further investigated using isolates from 1994 on wheat, oats and barley in cross-infection pathogenicity tests using two winter wheat cultivars with low and high resistance ratings for Fusarium.
Materials and methods
Isolates of M. nivale were obtained from the 1994 harvest (71 isolates) and the 1996 harvest (152 isolates). The single-spore isolates used were plated out on PDA and incubated at 18°C under a 12-h cycle of NUV for 10 ;days. Fifty conidia of each isolate were examined and spore length and number of septa calculated. Isolates were then categorized into two groups and confirmed by RAPD-PCR. Pathogenicity was assessed using a detached leaf method with cvs Encore and Spark as the wheat hosts. Segments 5-cm long were placed on 0.5% water agar containing 10 ;mg/l kinetin and inoculated with a 10 ;µl droplet. Wheat, oat and barley isolates of M. nivale var. nivale and M. nivale var. majus were used. Incubation period and lesion area were measured as indicators of pathogenicity (aggressiveness).
Results and conclusions
Using RAPD profiles, of the 71 isolates that were examined from the 1994 harvest, 44 (62%) showed a banding pattern of var. majus group and the remaining 27 isolates (38%) showed a greater degree of variation which corresponded to the var. nivale group. In the 1996 harvest, of the 152 isolates examined, 93 (61%) were classified as var. majus and 59 (39%) classified as var. nivale. Conidial morphology differed between isolates. Mean conidial length was 12-21.6 ;µm. Conidia with a width of 4.8 ;µm were classified as var. majus; those with <4.8 ;µm as var. nivale. The majority of isolates from wheat and barley cultivars in both the 1994 and 1996 harvests were classified as M. nivale var. majus and from oats as M. nivale var. nivale. Predominance of one variety over another may be due to climate, cropping history or source of seed, or a differential sensitivity to chemical seed dressing. In this survey M. nivale var. majus was more common than M. nivale var. nivale on grain tested. In a recent large-scale survey of isolates from wheat tissue in the UK, 93% of grain isolates were var. majus and only 7% were var. nivale. Almost all of the oat isolates tested were of the var. nivale group. This may suggest that var. majus survives better on wheat and barley than var. nivale and has a better competitive ability or is more efficiently seed transmitted. It could also indicate real differences in the abundance of each fungal variety. Results from detached leaf tests indicated var. majus to be more aggressive than var. nivale on wheat, irrespective of whether isolates came from wheat, barley or oats.
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