Opportunities and challenges for improved management of foliar pathogens in annual clover pastures across southern Australia

Abstract

Foliar pathogens result in significant losses in herbage and seed yields and regeneration capacity in annual clover pastures, the last leading to their rapid deterioration and lack of persistence. The most important pathogens include Kabatiella caulivora (clover scorch), Cercospora zebrina (cercospora), Uromyces trifolii-repentis (rust), Erysiphe trifoliorum (powdery mildew), and Leptosphaerulina trifolii (pepper spot). Several other foliar pathogens on annual clovers, in particular Phoma medicaginis (black stem and leaf spot), one or more Stemphylium spp. (stemphylium leaf spot), Pseudopeziza trifolii (common leaf spot), Stagonospora spp. (stagonospora leaf spot), Colletotrichum trifolii (anthracnose) and Sclerotinia trifoliorum (sclerotinia), occur widely and together contribute to reduce productivity in some localities. Severe attack by the most important pathogens (e.g. K. caulivora, U. trifolii-repentis, E. trifoliorum) not only greatly reduces winter–spring pasture production but frequently also coincides with the critical feed shortage across autumn–winter, leading to significantly decreased autumn–winter biomass production in regenerating stands. Approaches to disease control include a range of management strategies. Wider utilisation of cultural and fungicidal control strategies offers producers greater management flexibility, particularly in conjunction with deployment of cultivars with useful resistance. Host resistance offers the greatest potential for delivering the most cost-effective and long-term control. Many of these foliar pathogens co-occur, magnifying losses; this highlights the need for individual host genotypes with resistance to multiple pathogens and unique geographic locations such as Sardinia offer enormous scope to select such clovers. Future research opportunities and critical priorities to improve management of foliar pathogens in annual clover pastures across southern Australia include the need to: (i) define pathogen strain–race structures, particularly for K. caulivora and U. trifolii-repentis, and determine associated host resistances against specific strains–races to allow strategic deployment of host resistances; (ii) define relative resistances to major fungal foliar pathogens of all parental and near-release breeding genotypes and all commercial cultivars across important annual clover species; (iii) identify new sources of host resistance, particularly genotypes with cross-resistance to multiple pathogens, for breeders to utilise; (iv) identify and demonstrate the benefits to farmers of effective cultural (e.g. grazing, removal of infested residues) and fungicidal control options that allow greater management flexibility to reduce the impact of fungal foliar diseases; and (v) determine current incidences and impacts (losses and economic importance) of major fungal foliar diseases in the different agro-climatic regions across southern Australia. Failure to address these critical issues leaves livestock industries carrying the risks from release of new varieties of unknown susceptibilities to one or more of the major foliar diseases, and the risks from continued use of older varieties exposed to new pathogen races; with few if any flexible management options during periods of critical feed shortage; and without the basic information on current disease impacts that is needed to make sensible management and funding decisions.