5.5.14
EFFECT OF TEBUCONAZOLE IN PLANTA ON THE FITNESS OF RHYNCHOSPORIUM SECALIS STRAINS DIFFERING IN SENSITIVITY TO TEBUCONAZOLE IN VITRO

B ROBBERTSE1, C LENNOX2 and PW CROUS2

lARC-Small Grain Institute, Private Bag X29, Bethlehem 9700, South Africa; 2Department of Plant Pathology, University of Stellenbosch, Private Bag Xl, Matieland 7602, South Africa

Background and objectives
Barley scald caused by Rhynchosporium secalis is the most important disease of barley in the Western Cape Province of South Africa [1]. Tebuconazole has been used as a foliar spray against scald since 1989. In a previous study the tebuconazole sensitivity of R. secalis isolates collected throughout the barley producing area were monitored in vitro by determining the minimum inhibitory concentration (MIC). The aim of the present study was to evaluate the fitness of these characterised isolates with respect to lesion development and sporulation on plants treated with fungicide, and hence correlate their in vitro fungicide sensitivity with their ability to reproduce in planta. Development of resistance to fungicides is influenced by the fitness of populations to all the factors favourable to the production of progeny. Studying factors such as lesion size and sporulation will therefore give some indication of the risk of resistance development against tebuconazole in the South African R. secalis population.

Materials and methods
In a previous study isolates of were characterised in vitro towards tebuconazole, and the following classes determined: less sensitive (MIC = 30pg/mi), intermediate (MIC = 3 [jglmi) and sensitive (MIC = 0.1 pg/mi). Two isolates of each class were inoculated onto the susceptible barley cultivar, Clipper. The percentage leaf area diseased and sporulation ability (on 5 crn2 of diseased leaf) of each strain were determined on unsprayed and tebuconazole treated barley leaves with tebuconazole being applied at the rate recommended to barley growers in South Africa (416 mg a.i./1L). The data was processed in a randomised block analysis of variance in which contrasts were used to detect differences. P < 0.05 indicated a significant difference.

Results and conclusions
Typical barley scald lesions were produced on unsprayed barley leaves by all the R. secalis isolates, resulting in the total loss of turgor and collapse of inoculated leaves. Differences in lesions size could therefore not be detected between the different R. secalis isolates compared. On unsprayed plants the mean sporulation/mi of less sensitive isolates was not significantly different to that of sensitive isolates (P = 0.5764). Furthermore, no significant difference was detected in the sporulation of either sensitive (P = 0.1519) or less sensitive (P = 0.7538) isolates on sprayed and unsprayed leaves. Tebuconazole, therefore, did not have a significant influence on the sporulation of sensitive isolates or isolates that were characterised as being 300 times less sensitive. The mean percentage area covered by lesions were significantly higher (49% versus 25%; P = 0.0003) for less sensitive isolates than sensitive isolates on fungicide treated plants. Although less sensitive isolates were able to induce significantly more disease than sensitive isolates, some level of control was still obtained. Isolates with an in vitro MIC of 30 pg/mi could therefore not be described as resistant to tebuconazole. In general no influence on the sporulation ability of isolates with varying in vitro sensitivity to tebuconazole was detected between sprayed and unsprayed leaves. However, there was some indication that lesion development of isolates at baseline sensitivity was more prominently influenced by the presence of tebuconazole than isolates 300 times less sensitive in vitro.

References
1. Trench TN, Wilkinson DJ and Esterhuysen SP, 1992. South African Plant Disease Control Handbook. Farmer Support Group, University of Natal, Pietermaritzburg, South Africa. 534 pp.