A HISTOLOGICAL COMPARISON OF COMPATIBLE AND INCOMPATIBLE INTERACTIONS BETWEEN SUBTERRANEAN CLOVER AND TWO RACES OF KABATIELLA CAULIVORA
KL BAYLISS1,5, J KUO2, K SIVASITHAMPARAM1,5, M BARBETTI3,5 and ES LAGUDAH4
1Department of Soil Science and Plant Nutrition, Faculty of Agriculture, University of Western Australia, Nedlands, WA 6907, Australia; 2Centre for Microscopy and Microanalysis, University of Western Australia, Nedlands, WA 6907, Australia; 3Agriculture Western Australia, South Perth, WA 6151, Australia; 4CSIRO Plant Industry, Canberra, ACT 2601, Australia; 5Centre for Legumes in Mediterranean Agriculture, University of Western Australia, Nedlands, WA 6907, Australia
Background and objectives
Until recently, the disease was controlled with resistant varieties. However, in 1990, a 'new', resistance-breaking race emerged and caused severe disease on some previously highly resistant varieties of subterranean clover in Western Australia . We examined the infection process of the 'new' race in comparison to that of the 'old' race on two varieties of subterranean clover (T. subterraneum).
Materials and methods
Results and conclusions
Plants susceptible to K. caulivora developed large, water-soaked lesions. In contrast, on resistant plants the lesions were restricted to minute black specks. The 'new' race also caused large lesions to develop on Daliak (previously resistant). Loss of chlorophyll and starch grains in cells ahead of invading hyphae was the first evidence of infection. A hyphal network then developed inter- and intracellularly, before cell lysis occurred. Cells in uninfected plants stained positively for hemicellulose and pectin. Negative staining occurred in cells infected with the pathogen, indicating the degradation and loss of structural components of the cell wall. Both races infected all tissues in leaves, petioles and petiolules in susceptible plants. Only the 'new' race could extensively invade the variety previously resistant to K. caulivora (Daliak), in which infection by the old race was limited to the epidermal cells and two to three layers of mesophyll cells. Histochemical staining did not reveal any distinct plant resistance responses in leaves, petioles or petiolules.
Apart from the resistance of Daliak to the old race, there was no difference in the infection process of K. caulivora on subterranean clover for either race. It is possible that infection of Daliak by the new race is a consequence of the plant failing to recognise this new race, thus preventing or delaying the resistance response.