H. Owen

Studies on leaf blotch of barley (Rhynchosporium secalis)

In seedling tests, some isolates of Rhynchosporium secalis attacked all cultivars of barley tested, but others, though able to attack most cultivars were unable to attack Dea and Pioneer. This suggests that there were two physiologic races of the fungus among the fifteen isolates tested. In addition to their specific resistance to the second race, Dea and Pioneer were partially resistant to all isolates, as were (to varying degrees) some of the other cultivars not specifically resistant to either race. Though there were some discrepancies, cultivars generally behaved in the same way in seedling tests as in the field. Though isolates of the fungus differed in the rate at which their spores germinated, any one isolate germinated similarly on all cultivars of the host. Resistance in the host was expressed at the stage of penetration. Leaves of Dea and Pioneer were penetrated only to a very slight extent by isolates to which they were specifically resistant, and were penetrated less than leaves of Cambrinus and Proctor by isolates to which they had non-specific resistance. Although in general the extent to which mycelium proliferated below the cuticle was correlated with the severity of symptoms, Dea and Pioneer have non-specific resistance partly because in them more mycelium is associated with any particular level of severity of external symptoms than in Cambrinus and Proctor. Some cultivars have non-specific resistance because on them spores are less readily formed.

Physiologic races of Rhynchosporium secalis on barley in Britain

By using newly developed techniques, 12 varieties of barley were inoculated with 122 single spore isolates of Rhynchosporium secalis collected from British barley crops in 1967. Two distinct physiologic races were identified. U.K. 1 was the predominant race (96 isolates); it infected only one out of the 12 varieties (Cambrinus), whereas race U.K. 2 (26 isolates) infected nine varieties. Both races were widely distributed in Britain.

Resistance of barley varieties to establishment of subcuticular mycelia by Rhynchosporium secalis

Spores of Rhynchosporium secalis (Oudem.) J. J. Davis established subcuticular hyphae in leaves of both susceptible and resistant barley varieties. The frequency, and rate of extension, of hyphae was greater in susceptible than in resistant varieties. The resistance of barley varieties to the establishment and growth of hyphae was associated with the presence of previously identified major genes. Cuticle thickness did not contribute to this resistance which was, at least in part, an active response to invasion. Isolates of Race U.K. 2 were more effective than isolates of U.K. 1 in overcoming the resistance to hyphal establishment of any variety.

Transmission of Rhynchosporium secalis on barley grain

Lesions of Rhynchosporium secalis (Oudem.) J. J. Davis on barley grains in samples from the 1969 crop had the capacity to transmit infection to barley leaves and to daughter seedlings. Field trials of the samples indicated that seed-borne infections were a source of primary inoculum for leaf blotch disease.

Physiologic specialization in Rhynchosporium secalis

British isolates of Rhynchosporium secalis show strict host specialization. Five isolates from Argentina and one from Australia behave similarly. Differences in pathogenicity are not apparently correlated with morphological characters of the spores, nor, except to a limited degree, with cultural features.

Physiologic races of Rhynchosporium secalis on cultivated barley

Ten British isolates of R. secalis from cultivated barley could be arranged in eight groups according to their pathogenicity to seven barley cultivars. One isolate was conspicuously different from the others, both in range of cultivars attacked and in aggressiveness.

Susceptibility of barley cultivars to leaf blotch and aggressiveness of Rhynchosporium secalis races

Basal segments from the second seedling leaves of four spring barley cultivars were inoculated with five race UK1 isolates and four race UK2 isolates of the leaf blotch fungus Rhynchosporium secalis (Oudem.) J.J. Davis. Distinct differences between cultivars in symptom development allowed a ranking of susceptibility similar to that obtained in numerous field tests. Four of the five race UK1 isolates were more aggressive than the four race UK2 isolates.

Factors influencing spore germination in Rhynchosporium secalis.

Spores of Rhynchosporium secalis (Oudem.) J. J. Davis produced a self-inhibitor of germination, the effectiveness of which was increased by increasing concentration, and decreased by the increasing availability to the spores of exogenous nutrients. Barley leaves provided a source of exogenous nutrients irrespective of their susceptibility to Rhynchosporium (leaf blotch).

Host range of Rhynchosporium secalis

Two degrees of physiologic specialization were found in isolates of Rhynchosporium secalis (Oudem.) J. J. Davis. All isolates from barley, and one each from Agropyron repens (L.) Beauv. and rye, were pathogenic to their original host only; other isolates from rye and three grasses were pathogenic to their original host and to barley. The numbers of subcuticular hyphae established in barley by isolates from each original host were of the same order. After serial passage through barley leaves, isolates from other hosts were more aggressive to barley; however, an isolate from barley became less aggressive.