Cryptic diversity found in Didymellaceae from Australian native legumes

Ascochyta koolunga (Didymellaceae, Pleosporales) was first described in 2009 (as Phoma koolunga) and identified as the causal agent of Ascochyta blight of Pisum sativum (field pea) in South Australia. Since then A. koolunga has not been reported anywhere else in the world, and its origins and occurrence on other legume (Fabaceae) species remains unknown. Blight and leaf spot diseases of Australian native, pasture and naturalised legumes were studied to investigate a possible native origin of A. koolunga. Ascochyta koolunga was not detected on native, naturalised or pasture legumes that had leaf spot symptoms, in any of the studied regions in southern Australia, and only one isolate was recovered from P. sativum. However, we isolated five novel species in the Didymellaceae from leaf spots of Australian native legumes from commercial field pea regions throughout southern Australia. The novel species were classified on the basis of morphology and phylogenetic analyses of the internal transcribed spacer region and part of the RNA polymerase II subunit B gene region. Three of these species, Nothophoma garlbiwalawardasp. nov., Nothophoma naiawusp. nov. and Nothophoma ngayawangsp. nov., were isolated from Senna artemisioides. The other species described here are Epicoccum djirangnandirisp. nov. from Swainsona galegifolia and Neodidymelliopsis tinkyukukusp. nov. from Hardenbergia violacea. In addition, we report three new host-pathogen associations in Australia, namely Didymella pinodes on S. artemisioides and Vicia cracca, and D. lethalis on Lathyrus tingitanus. This is also the first report of Didymella prosopidis in Australia.

Didymella pinodes Affects N and P Uptakes and Their Efficiencies in a Tripartite Mutualism of Pea

In pea (Pisum sativum L.) production, Didymella pinodes (Berk. & A. Bloxam) Petr. is the most damaging aerial pathogen globally. In two completely randomized pot experiments with four replicates, we studied the effects of D. pinodes infection interaction with three symbiotic treatments (Rhizobium leguminosarum biovar viciae, arbuscular mycorrhizal fungi (AMF) and co-inoculation of both) and a non-symbiotic control on one or two pea cultivars. Grain yield and yield components of pea, uptakes and physiological efficiencies of N and P and nitrogen fixation were recorded. The results show that there were significant interaction effects among treatments. Therefore, productivity of crops and their uptakes and efficiencies of N and P are dependent on plant health conditions, effectiveness of microbial symbionts and response of pea genotypes. For cv. Protecta inoculated with both symbionts, pathogen infection compared to healthy plants significantly enhanced P acquisition. Overall, plants inoculated with rhizobia alone had higher grain yield by 20–30% and nitrogen fixation by 20–25% than in dual symbiosis independent of plant health conditions. In conclusion, aerial pathogen, pea genotypes and microbial symbionts interactions modified N and P uptake and their efficiencies, which can lead to improving final grain yield quantity and quality in a sustainable farming system.

Aggressiveness Changes in Populations of Didymella pinodes over Winter and Spring Pea Cropping Seasons

ABSTRACTAscochyta blight, caused by the necrotrophic ascomyceteDidymella pinodes, is responsible for severe losses in winter and spring pea crops. Despite different climatic conditions, epidemics on winter and spring crops are due to a single population ofD. pinodes, suggesting gene flow either between the two crops or from reservoir sources during the cropping season. This should lead to similar pathogenicity characteristics in isolates sampled from the two crops. However, these hypotheses have never been formally tested. We therefore sampled a total of 520D. pinodesstrains throughout a growing season from winter and spring pea plots (WP and SP, respectively) and from winter and spring trap plants (TWP and TSP). Amplified fragment length polymorphism (AFLP) markers revealed high genetic diversity within subpopulations, whereas pathogenicity tests showed that mean aggressiveness increases over the course of an epidemic. These results support the idea that alloinoculum contributes to the carryover of epidemics between winter and spring crops and that the most aggressive isolates are selected as an epidemic progresses.IMPORTANCEAscochyta blight, caused byDidymella pinodes, is responsible for severe losses in pea crops. While previous studies have shown that ascochyta blight epidemics on winter and spring crops are due to a single population ofD. pinodes, suggesting that isolates from the two crops present similar pathogenicity characteristics, that hypothesis have never been tested. Genetic analysis of subpopulations sampled throughout a growing season from winter and spring pea plots revealed high genetic diversity within subpopulations, whereas pathogenicity tests showed that mean aggressiveness increases over the course of an epidemic.

Relative Host Resistance to Black Spot Disease in Field Pea (Pisum sativum) is Determined by Individual Pathogens

Black spot, also known as Ascochyta blight, is the most important disease on field pea (Pisum sativum). It is caused by a complex of pathogens, the most important of which in Australia include Didymella pinodes, Phoma pinodella, and P. koolunga. The relative proportions of these and other component pathogens of the complex fluctuate widely across time and geographic locations in Australia, limiting the ability of breeders to develop varieties with effective resistance to black spot. To address this, 40 field pea genotypes were tested under controlled environment conditions for their individual stem and leaf responses against these three pathogens. Disease severity was calculated as area under disease progress curve (AUDPC), and subsequently converted to mean rank (MR). The overall rank (OR) for each pathogen was used to compare response of genotypes under inoculation with each pathogen. The expressions of host resistance across the field pea genotypes were largely dependent upon the individual test pathogen and whether the test was on stem or leaf. Overall, P. koolunga caused most severe stem disease; significantly more severe than either D. pinodes or P. pinodella. This is the first report of the host resistance identified in field pea to P. koolunga; the five genotypes showing highest resistance on stem, viz. 05P778-BSR-701, ATC 5338, ATC 5345, Dundale, and ATC 866, had AUDPC MR values <250.4, while the AUDPC MR values of the 19 genotypes showing the best resistance on leaf was less than 296.8. Two genotypes, ATC 866 and Dundale, showed resistance against P. koolunga on both stem and leaf. Against D. pinodes, the four and 16 most resistant genotypes on stem and leaf had AUDPC MR values <111.2 and <136.6, respectively, with four genotypes showing resistance on both stem and leaf including 05P770-BSR-705, Austrian Winter Pea, 06P822-(F5)-BSR-6, and 98107-62E. Against P. pinodella, four and eight genotypes showing the best resistance on stem and leaf had AUDPC MR values <81.3 and <221.9, respectively; three genotypes, viz. 98107-62E, Dundale, and Austrian Winter Pea showed combined resistance on stem and leaf. A few genotypes identified with resistance against two major pathogens of the complex will be of particular significance to breeding programs. These findings explain why field pea varieties arising from breeding programs in Australia fail to display the level or consistency of resistance required against black spot and why there needs to be a wider focus than D. pinodes in breeding programs.

Temporal and Spatial Changes in the Pea Black Spot Disease Complex in Western Australia

Black spot (also referred to as Ascochyta blight, Ascochyta foot rot and black stem, and Ascochyta leaf and pod spot) is a devastating disease of pea (Pisum sativum) caused by one or more pathogenic fungi, including Didymella pinodes, Ascochyta pisi, and Phoma pinodella. Surveys were conducted across pea-growing regions of Western Australia in 1984, 1987, 1989, 1996, 2010, and 2012. In total, 1,872 fungal isolates were collected in association with pea black spot disease symptoms. Internal transcribed spacer regions from representative isolates, chosen based on morphology, were sequenced to aid in identification. In most years and locations, D. pinodes was the predominant pathogen in the black spot complex. From 1984 to 2012, four new pathogens associated with black spot symptoms on leaves or stems (P. koolunga, P. herbarum, Boeremia exigua var. exigua, and P. glomerata) were confirmed. This study is the first to confirm P. koolunga in association with pea black spot symptoms in field pea in Western Australia and show that, by 2012, it was widely present in new regions. In 2012, P. koolunga was more prevalent than D. pinodes in Northam and P. pinodella in Esperance. P. herbarum and B. exigua var. exigua were only recorded in 2010. Although A. pisi was reported in Western Australia in 1912 and again in 1968 and is commonly associated with pea black spot in other states of Australia and elsewhere, it was not recorded in Western Australia from 1984 to 2012. It is clear that the pathogen population associated with the pea black spot complex in Western Australia has been dynamic across time and geographic location. This poses a particular challenge to development of effective resistance against the black spot complex, because breeding programs are focused almost exclusively on resistance to D. pinodes, largely ignoring other major pathogens in the disease complex. Furthermore, development and deployment of effective host resistance or fungicides against just one or two of the pathogens in the disease complex could radically shift the make-up of the population toward pathogen species that are least challenged by the host resistance or fungicides, creating an evolving black spot complex that remains ahead of breeding and other management efforts.