Tan spot disease under the lenses of plant pathologists

Necrotrophic plant pathogens pose an important threat to crop production, and many fungi in the Pleosporales have caused the sudden emergence of major epidemics on cereal crops. Tan spot of wheat, caused by Pyrenophora tritici-repentis, is one example, and since its emergence in the 1970s, scientists have explored its virulence and interactions with the host. In this chapter, our aim is to provide a comprehensive review of the most significant landmarks in tan spot research over the past 50 years from a plant pathology perspective.

Evaluation of a Novel Molecular Marker Associated with the Tan Spot Disease Response in Wheat

After nearly 40 years of DNA molecular marker development in plant breeding, the wheat research community has amassed an extensive collection of molecular markers which have been widely and successfully used for selection of agronomic, physiological and disease resistance traits in wheat breeding programs. Tan spot is a major fungal disease of wheat and a significant global economic challenge and is caused by the necrotrophic fungal pathogen Pyrenophora tritici-repentis (Ptr). Here, the potential for using a PCR-based marker (Ta1AS3422) present on the short arm of wheat chromosome 1A, was evaluated for effectiveness in distinguishing tan spot disease susceptibility. The marker was initially screened against 40 commercial Australian hexaploid wheat varieties, and those that amplified the marker had an overall lower disease score (2.8 ± 0.7 for seedlings and 2.4 ± 0.4 for plants at the tillering stage), compared to those lacking the marker which exhibited a higher disease score (3.6 ± 0.8 for both growth stages). The potential of Ta1AS3422 as a marker for the tan spot disease response was further assessed against a panel of 100 commercial Australian hexaploid wheat varieties. A significant association was observed between marker absence/presence and tan spot disease rating (Pearson’s chi-squared test, χ2 (6) = 20.53, p = 0.002), with absence of Ta1AS3422 associated with susceptibility. This simple and cost-effective PCR-based marker may be useful for varietal improvement against tan spot, although further work is required to validate its effectiveness.

Evaluation of Pyrenophora tritici-repentis Infection of Wheat Heads

The incidence of wheat head infection by Pyrenophora tritici-repentis (Ptr), the etiological agent of tan spot disease, was evaluated during grain development in a glasshouse experiment. Heads artificially inoculated with a Ptr spore suspension developed widespread brown spots across the spikelets, and mycelia and conidophores were observed on glumes and awns. Seeds of heavily infected heads were darkened and shrivelled, but no red smudge symptoms were apparent. The recovery rate of Ptr isolates from the inoculated wheat heads was low, and colonies that were re-isolated displayed an irregular morphology with reddish mycelia when grown on agar plates. The presence of Ptr on inoculated wheat heads was assessed directly via PCR detection, and a limitation of Ptr hyphae to proliferate beyond the point of contact of spore inoculum on floret tissues was observed. The systemic transmission of Ptr from infected seeds was minimal; however, saprophytic growth of the pathogen occurred on the senescing leaves of wheat plants grown from inoculated seeds. Thus, even though Ptr seed infection is not as common as foliar infection, infected seeds are still a source of disease inoculum and screening for pathogen contamination is advisable.

Characterizing Virulence of the Pyrenophora tritici-repentis Isolates Lacking Both ToxA and ToxB Genes

The fungus Pyrenophora tritici-repentis (Ptr) causes tan spot of wheat crops, which is an important disease worldwide. Based on the production of the three known necrotrophic effectors (NEs), the fungal isolates are classified into eight races with race 4 producing no known NEs. From a laboratory cross between 86–124 (race 2 carrying the ToxA gene for the production of Ptr ToxA) and DW5 (race 5 carrying the ToxB gene for the production of Ptr ToxB), we have obtained some Ptr isolates lacking both the ToxA and ToxB genes, which, by definition, should be classified as race 4. In this work, we characterized virulence of two of these isolates called B16 and B17 by inoculating them onto various common wheat (Triticum aestivum L.) and durum (T. turgidum L.) genotypes. It was found that the two isolates still caused disease on some genotypes of both common and durum wheat. Disease evaluations were also conducted in recombinant inbred line populations derived from two hard red winter wheat cultivars: Harry and Wesley. QTL mapping in this population revealed that three genomic regions were significantly associated with disease, which are different from the three known NE sensitivity loci. This result further indicates the existence of other NE-host sensitivity gene interactions in the wheat tan spot disease system.

Evaluation of spray nozzles for fungicidal control of tan spot in wheat

Tan spot, caused by Pyrenophora tritici-repentis (Died.) Drechs, is a serious constraint on wheat yields in the Southern Cone region of South America. A 2-year experiment was conducted to evaluate fungicide deposition, disease development and grain yield. Three spray nozzles were evaluated: an air-induction flat fan (AI), a wide-angle flat fan (TT), and an extended-range flat fan (XR). A systemic fungicide containing azoxystrobin and cyproconazole was used in both years. Tan spot severity and the area under non-green leaf area disease progress curve (AUNGLA) were analysed. There were no significant differences in deposition among nozzles, and no significant interactions between nozzles and leaf layers in the first year. In both years, AUNGLAs were similar for the three nozzles types, and the tan spot severity in untreated plots was significantly higher than in fungicide-applied plots. Grain yield was higher in the fungicide-applied plots, and there were no significant differences among nozzles in both years. Droplet size had no effect on the fungicide’s efficacy for tan spot control in Uruguay across three susceptible wheat cultivars. The use of drift-reducing nozzles and a systemic fungicide in these trials led to satisfactory performance for spray deposition, canopy penetration and control of the tan spot disease of wheat in the same way as expected from conventional nozzles.