•Improved levels of partial host resistance identified offer new management prospects.•Improved resistance offers renewed opportunities for cultural and fungicidal options.•New research insights ...highlight potential for further improvements in host resistance.•Future ‘stacking’ of broad antifungal genes offers even more effective control.
Didymella pinodes is the major pathogen of the ascochyta blight disease complex of field pea. The disease is endemic in all major field pea producing countries, frequently causing heavy losses in yield and quality. It is the most challenging of field pea diseases to manage, with most fungicides only partially effective and/or not cost-effective. In the absence of effective levels of host resistance, historically, the best management option has been delayed sowing and/or crop rotation to avoid major ascospore showers, but delayed sowing generally incurs a concurrent heavy yield penalty. This review evaluates world-wide progress in understanding critical components of black spot in terms of its management and evaluates opportunities both for new research and for development of more effective and sustainable management of this disease, using improved host resistance as a foundation to build and deploy more effective integrated disease management strategies. In the past decade, research has provided considerable new insights into potential ascochyta blight management strategies, including new insights into inheritance of host resistance, response to selection, and use of molecular technology, that together have demonstrated the potential to improve the level of host resistance. Significant improvements have been reported in the level of partial resistance of field pea with improved agronomic traits. Consequently, while the effect of this level of resistance in reducing D. pinodes infection remains to be quantified, for the first time such novel germplasm offers the prospect of revising and reintegrating the different disease management options at the farm level, based on deployment of such resistance. Combining this improved host resistance with both cultural management options and fungicidal application offers new opportunities. It is likely that previously restrictive cultural management and ineffective fungicidal control measures will re-emerge as effective and profitable practices when used in conjunction with partially-resistant germplasm. While this approach is an effective first stage to better manage ascochyta blight, future ‘stacking’ of broad antifungal genes on current moderately resistant varieties using genomic tools and/or GM technologies offers an avenue for even more effective control of D. pinodes.
The value of Katanning Early Maturing (KEM) breeding lines from Western Australia, derived from Brassica napus x B. juncea crosses, was assessed as a source of germplasm for resistance to blackleg ...disease (caused by Leptosphaeria maculans) in spring-type oilseed rape cultivars. The stability of blackleg resistance in these KEM lines was related to key cytological characteristics to determine why there are poor levels of introgression of this resistance into progeny. Promising recombinant KEM lines were crossed with the spring-type B. napus cv. Dunkeld, which has useful polygenic resistance to blackleg, and screened for resistance. The lines were analyzed cytologically for pairing of bivalents in each generation to aid in the selection of stable recombinant lines. KEM recombinant lines showing regular meiotic behavior and a high level of blackleg resistance were obtained for the first time. We also showed that the stable introgression of the B. juncea resistance from the KEM lines into a 'Dunkeld' background was possible. Inoculation of selfing and backcross populations with isolates of L. maculans having different AvrLm genes indicated that the B. juncea resistance gene, Rlm6, had been introgressed into a B. napus spring-type cultivar carrying polygenic resistance. The combination of both resistances would enhance the overall effectiveness of resistance against L. maculans. This is clearly needed in Australia and France where cultivars relying upon single dominant gene-based resistance for their effectiveness have proved not durable.
The decline/death of strawberry plants is considered to be a serious challenge to strawberry production in Western Australia (WA). Although crown and root diseases have been found to be important ...contributing factors, little is known on the extent of crown and root diseases or the associated fungal and oomycete pathogens. Surveys were undertaken in 2008 to determine the severity of crown and root diseases in commercial strawberry fields in WA, to determine the influence of cropping history, fumigation, soil nutrients and pH on severity of such diseases, and to identify the associated pathogens. Among the sites surveyed, there was a significant variation with the decline/death index (%DI) ranging from 2.9 to 39.7. The level of strawberry plant decline/death progressed significantly from August to October, with the %DI rising from 13.0 to 39.2; There was a significant difference in strawberry plant decline/death between fumigated and non-fumigated beds, with a %DI of 7.1 for fumigated and 45.2 for non-fumigated beds. However, it was evident that in situations where there were periods of 3 or more years involving crop rotations between successive strawberry crops that the level of plant decline/death was lower, even in the absence of fumigation. Both the severity of crown disease and root disease were positively correlated with that of strawberry plant decline/death. There was a significant negative correlation between soil pH and the decline/death of strawberry plants. The main pathogens associated with crown and root diseases of strawberry were
Fusarium oxysporum
,
Rhizoctonia
spp. (AG-A, AG-C, AG-I, AG-K and others)
, Cylindrocarpon destructans
,
Phoma exigua
,
Gnomonia fructicola
,
Phytophthora cactorum
,
Pythium ultimum
and
Macrophomina phaseolina
.
F. oxysporum
was most frequently isolated from crowns, at a frequency of 41.2%.
Rhizoctonia
spp. and
C. destructans
were most frequently isolated from roots, at a frequency of 11.8% and 12.0%, respectively. This study demonstrates, for the first time, that strawberry production in WA is severely compromised by crown and root diseases, especially late in the production season, and impacts are clearly greater in non-fumigated field beds as well as those with relatively lower soil pH (4.5 ~ 6 in CaCl
2
). This study also demonstrates that
F. oxysporum
is the major pathogen associated with crown disease, and
Rhizoctonia
spp
.
are important pathogens associated with root disease of strawberry in WA.
Necrotrophic pathogens of the cool season food legumes (pea, lentil, chickpea, faba bean and lupin) cause wide spread disease and severe crop losses throughout the world. Environmental conditions ...play an important role in the development and spread of these diseases. Form of inoculum, inoculum concentration and physiological plant growth stage all affect the degree of infection and the amount of crop loss. Measures to control these diseases have relied on identification of resistant germplasm and development of resistant varieties through screening in the field and in controlled environments. Procedures for screening and scoring germplasm and breeding lines for resistance have lacked uniformity among the various programs worldwide. However, this review highlights the most consistent screening and scoring procedures that are simple to use and provide reliable results. Sources of resistance to the major necrotrophic fungi are summarized for each of the cool season food legumes. Marker-assisted selection is underway for Ascochyta blight of pea, lentil and chickpea, and Phomopsis blight of lupin. Other measures such as fungicidal control and cultural control are also reviewed. The emerging genomic information on the model legume, Medicago truncatula, which has various degrees of genetic synteny with the cool season food legumes, has promise for identification of closely linked markers for resistance genes and possibly for eventual map-based cloning of resistance genes. Durable resistance to the necrotrophic pathogens is a common goal of cool season food legume breeders.
Rice blast caused by Magnaporthe oryzae is the most destructive disease of rice worldwide. Development of resistant varieties is considered as the most cost‐effective and sustainable way to manage ...rice blast. However, there remains a lack of knowledge about the resistance of rice varieties to blast disease in Australia. This study was conducted to determine if there was any resistance existing among the rice varieties grown in Australia to M. oryzae isolates from this country that belong to different races. There was a resistant reaction of the variety SHZ‐2 to all the five races of IA‐1, IA‐3, IA‐63, IB‐3 and IB‐59, with a percent disease index (%DI) less than 40. Varieties NTR587, BR‐IRGA‐409, Ceysvoni and Rikuto Norin 20 showed a resistant reaction to races IA‐3, IA‐63, IB‐3 and IB‐59; and the variety Kyeema exhibited a resistant reaction to races IA‐3, IB‐3 and IB‐59. For the races IA‐1 and IB‐59 with more than one isolate, varieties with differential disease reactions across different isolates belonging to the same race were also revealed: five varieties, Langi, Opus, Sherpa, Viet 1 and Topaz, exhibited differential disease reactions to the three IA‐1 isolates; 10 varieties showed differential disease reactions to the four IB‐59 isolates; in addition, the varieties that had differential disease reactions to the IA‐1 isolates also exhibited differential disease reactions to the IB‐59 isolates of race. This study provides valuable resistance sources for breeding programmes to develop rice varieties with resistance to multiple races of M. oryzae in Australia.
Studies on infection processes and gene expression were done to determine differential responses of cultivars of Trifolium subterraneum resistant and susceptible to infection by races of Phytophthora ...clandestina. In the infection process study, one race was inoculated onto the roots of T. subterraneum cvs. Woogenellup and Junee (compatible or incompatible interactions, respectively). There were no differences in relation to the processes of cyst attachment, germination, and hyphal penetration. There were, however, major differences in infection progression observed post-penetration between compatible and incompatible interactions. In susceptible cv. Woogenellup, hyphae grew into the vascular bundles and produced intercellular antheridia and oogonia in the cortex and stele by 4 days postinoculation (dpi), oospores in the cortex and stele by 8 dpi, when sporangia were evident on the surface of the root. Infected taproots were discolored. Early destruction of taproots prevented emergence of lateral roots. Roots of resistant cv. Junee showed no oospores or sporangia and no disease at 8 dpi. In the gene expression studies, two races of P. clandestina were inoculated onto three cultivars of T. subterraneum. Results showed that three genes known to be associated with plant defense against plant pathogens were differentially expressed in the roots during compatible and incompatible interactions. Phenylalanine ammonia lyase and chalcone synthase genes were activated 4 h postinoculation (hpi) and cytochrome P450 trans-cinnamic acid 4-monooxygenase gene was activated 8 hpi in the incompatible interactions in cvs. Denmark and Junee following inoculation with Race 177. In contrast, in compatible interactions in cv. Woogenellup, there were no significant changes in the activation of these three genes following inoculation, indicating that these three genes were associated with the expression of resistance to Race 177 of the pathogen by the host. To confirm this result, in the second test, cv. Woogenellup was challenged by Race 000 of P. clandestina. In this incompatible interaction, cv. Woogenellup was resistant and expressed highly all three genes in the manner similar to the incompatible interactions observed in the first test.
Specific resistance loci in plants are generally very efficient in controlling development of pathogen populations. However, because of the strong selection pressure exerted, these resistances are ...often not durable. The probability of a resistance breakdown in a pathosystem depends on the evolutionary potential of the pathogen which is affected by: (i) the type of resistance (monogenic and/or polygenic), (ii) the type of reproduction of the pathogen (sexual and/or asexual), (iii) the capacity of the pathogen for dispersal, (iv) the resistance deployment strategy (pyramiding of specific resistances, mixture of cultivars, spatio-temporal alternation), (v) the size of the pathogen population, which is affected by control methods and environmental conditions. We propose the concept of Integrated Avirulence Management (IAM) to enhance the durability of specific resistances. IAM involves a strategy to limit the selection pressure exerted on pathogen populations and, at the same time, reduce the size of pathogen populations by combining cultural, physical, biological or chemical methods of control. Several breakdowns of resistance specific to Leptosphaeria maculans, the causal agent of phoma stem canker have occurred in Europe and in Australia. This review paper examines control methods to limit the size of L. maculans populations and discusses how this limitation of population size can enhance the durability of specific resistances. It proposes pathways for the development of a spatially explicit model to define IAM strategies. Simulation results are presented to demonstrate the potential uses of such a model for the oilseed rape/L. maculans pathosystem.
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