Resistance to clover scorch disease (Kabatiella caulivora) among accessions of purple clover (Trifolium purpureum) and its relationship to the eco-geography of collection sites

Visual ratings of disease reaction to a mixture of races 1 and 2 of clover scorch (Kabatiella caulivora) were conducted on inoculated field plots of 206 accessions of Trifolium purpureum (191 var. purpureum and 15 var. pamphyllicum) collected from the Mediterranean basin and surrounding regions. Disease severity scores of the resistant check, cv. Denmark subterranean clover (T. subterraneum), were clearly differentiated from the susceptible check, cv. Paratta purple clover. Nearly 33% of the accessions were resistant to both races. Resistant plants tended to flower later and originate from higher latitudes, where K. caulivora is more widespread. The results of this investigation led to development of ELECTRA™, the first cultivar of purple clover with resistance to both races of K. caulivora.

Resistance to race 1 of Kabatiella caulivora in subterranean clover (Trifolium subterraneum L.) cultivars and breeding lines

Twenty-eight cultivars and 106 F6-derived breeding lines of subterranean clover (Trifolium subterraneum) were screened in the field for their response to clover scorch disease caused by race 1 of Kabatiella caulivora. Eleven of the cultivars, including Denmark and Goulburn, were classified as resistant. Breeding lines with Denmark parentage had 55% of progeny with resistance, while those of Goulburn had only 19% of resistant progeny, suggesting different modes of inheritance. Selection for resistance to race 2 of K. caulivora in the F4 generation markedly increased the probability of selecting F6-derived lines with resistance to race 1, suggesting linkage between genes for resistance to both races.

Coolamon subterranean clover (Trifolium subterraneum L. var. subterraneum)

Coolamon is a mid-season to late-season flowering F4-derived crossbred subterranean clover of var. subterraneum, developed by the collaborating organisations of the National Annual Pasture Legume Improvement Program. It is a replacement for Junee and has been selected for release on the basis of its greater herbage production and persistence, and its resistance to both known races of clover scorch. Coolamon is recommended for sowing in Western Australia, New South Wales, Victoria, South Australia and Queensland. It is best suited to well-drained, moderately acidic soils in areas with a growing season of 6.5–8 months that extends into November. Coolamon is best suited to phase farming and permanent pasture systems. It can also be used in cropping rotations, but at least 2 years of pasture are required between crops. Coolamon has been granted Plant Breeders Rights in Australia.

Variability within Kabatiella caulivora Race 1 and Race 2 revealed by cultural and molecular analyses

Kabatiella caulivora is the causal agent of clover scorch, a fungal disease of clover (Trifolium) species. Variability within and between K. caulivora Race 1 and Race 2 was determined by cultural characteristics, isozymes, and amplified fragment length polymorphisms (AFLP). Cultural studies indicated isolates from both races were highly variable. No differences were identified within or between races by isozyme analysis. Similarity coefficients, determined from AFLP analysis, indicated that isolates from different races were often more similar than isolates from the same race. Comparison of single representative isolates from Race 1 and Race 2, collected at a Denmark (Western Australia) disease site, with isolates collected from another site of clover scorch outbreak at Esperance, 300 km east of Denmark, indicated most of the isolates causing the second outbreak were similar to Race�2, confirming previously conducted pathogenicity tests. It is hypothesised that Race 2 may have evolved from Race 1, and that the level of variability in the pathogen indicates the potential for development of further new races of K. caulivora. The requirement for improved selection strategies, including the screening of new cultivars and breeding lines with multiple isolates of the pathogen, is discussed in relation to these findings.

Differences in symptom development in subterranean clover infected with Kabatiella caulivora Race 1 and Race 2 are related to host resistance

Clover scorch (Kabatiella caulivora) is a severe fungal disease of Trifolium spp. contributing to the collapse of pasture swards across southern Australia during warm, humid spring weather. Host plant responses associated with resistance to the disease were determined in 2 cultivars of subterranean clover (T. subterraneum) separately inoculated with K. caulivora Race 1 or Race 2. Germination of conidia of both races reached a maximum 5 days post-inoculation on cv. Woogenellup (susceptible to both races) and 4 days post-inoculation on cv. Daliak (resistant to Race 1 but susceptible to Race 2). Germ tube growth of Race 1 was inhibited on cv. Daliak and the percentage of conidia penetrating leaf surfaces was lowest on this race–cultivar combination. Susceptibility was characterised by large petiole lesions, with invasive hyphae extending through the mesophyll tissue into the pith and then through the phloem tissue of vascular bundles, eventually causing the petioles to collapse. Resistance was characterised by small, black lesions with invasive hyphae extending no further than the fourth layer of mesophyll cells. A suberin-based material was observed beneath infected mesophyll cells in the incompatible interaction, beyond which no further growth of hyphae occurred. Race 2 caused a faster rate of host tissue necrosis than Race 1 and also the breakdown of starch grains in uninvaded petiole tissues. Starch grains in plants infected with Race 1 were evenly distributed in uninvaded tissue. Sporulation was rarely observed in the incompatible interaction but was common in compatible interactions within 15 days post-inoculation. These responses to K. caulivora can now be used as a breeding tool in evaluating and selecting improved resistance to clover scorch disease among breeding lines of subterranean clover.

Breakdown in resistance of subterranean clovers to clover scorch disease (Kabatiella caulivora)

In spring 1990, in the Australasian Subterranean Clover and Alternative Legume Improvement Program field plots at Denmark, Western Australia, subterranean clover cultivars and crossbred lines normally highly resistant to clover scorch disease caused by Kabatiella caulivora, were severely affected by this disease. Testing the response of subterranean clover varieties to Kabatiella isolates taken from plants from these plots indicates the arrival of a new race of the fungus. The new race differs from all other isolates in overcoming the previously outstanding resistance of cultivars Green Range, Junee, Daliak and Esperance.

Variation in, and inheritance of seedling and adult-plant resistance in subterranean clover (Trifolium subterraneum L.) to leaf scorch (Kabatiella caulivora) (Kirch.) Karak

The disease reaction of seedlings (2 weeks old) from a field collection of subterranean clover (Trifolium subterraneum) to clover scorch (Kabatiella caulivora (Kirch.) Karak), revealed 89 immune plants out of a total of 1530. However, with adult plants (15-16 weeks old) only one plant remained immune. In the F2 population of the cross of the cultivars (Daliak X Woogenellup (D X W), seedling resistance was conditioned by a single dominant gene and in the cross (Enfield X Woogenellup) (EX W) by a single recessive gene. However, with adult plants, resistance in the (D X W) F2 was conditioned by a single recessive gene and that in the (EX W) F2 by two recessive genes. The effect of increasing age in reducing the proportion of plants resistant to clover scorch in both studies indicates that caution is needed in evaluating resistance to clover scorch in epidemiological, breeding and genetic studies.

Renovation of Yarloop subterranean clover pastures with Trikkala

Pastures containing the oestrogenic subterranean clover, Yarloop, were renovated and sown to Trikkala subterranean clover at the rate of 9 kg ha-1 germinable seed. The resultant pastures were continually grazed at either 7, 9 or 12 ewes ha for 6 years. Control Yarloop pastures were grazed similarly. Trikkala established at approximately 50 plants m-2 and represented 80% of all clover present in the year of sowing. Six years later Trikkala was successfully persisting at all stocking rates in terms of clover dry matter in spring (Trikkala 77%, Yarloop 23%), soil seed reserves in spring (Trikkala 450-1000 kg ha-1, Yarloop 300-450 kg ha-1) and plant density in spring (Trikkala 600-1000 plants m-2, Yarloop 200 plants m-2). Over the 6 years of the trial the proportion of Trikkala and Yarloop approached an equilibrium of 75% Trikkala to 25% Yarloop in all agronomic data collected. Compared with the control Yarloop pastures, the renovated Trikkala pastures maintained a greater clover percentage at all stocking rates. Available dry matter at the high stocking rate was consistently greater in the Yarloop pastures during winter and spring. At the low and medium stocking rates available dry matter tended to be greater in the Yarloop treatments for the first 2 years but was similar for both treatments in the latter 3 years. Clover scorch disease in the Yarloop pasture was controlled by spraying with a fungicide. Trikkala pastures were not sprayed and suffered minimal damage from the disease. Trikkala should perform better relative to Yarloop where clover scorch is a problem. It is concluded that Trikkala subterranean clover can successfully replace Yarloop subterranean clover in pastures previously dominated by Yarloop.

Relations between stocking rate, environment and scorch disease on grazed subterranean clover pasture in Western Australia

The effects of weather on clover scorch disease caused by Kabatiella caulivora, the effects of the disease on pasture and sheep production, and the effect of sheep stocking rate (6, 8, 10 and 12 sheep/ha) on the disease were studied in a field experiment over three seasons near Albany, Western Australia. Spraying with benlate was used to control clover scorch on some treatments. Mean temperatures during the period when symptoms of the disease increased were in the range 11-17�C. Rainfall was frequent each year during the same period. The rapid increase in disease symptoms was observed when pasture dry matter on offer was approximately 2500 kg/ha. Disease symptoms decreased during a period of water stress of the pasture. Clover scorch disease had no consistent effect on pasture or animal production except that pasture dry matter on offer in spring was reduced in the unsprayed treatments at the lower stocking rates in the second two years. Higher stocking rates (10 and 12 sheep/ha) reduced the level of clover scorch symptoms without the use of chemical control with benlate. At the period of peak scorch symptoms a stocking rate of 10 sheep/ha reduced disease incidence by 20-35% compared with 6 sheep/ha in unsprayed plots. Increased sheep stocking rates are suggested as a possible alternative to chemical control or replacement of susceptible strains of subterranean clover in grazed pasture.