Infection process of Stagonosporopsis tanaceti in pyrethrum seed and seedlings

Ray blight disease of pyrethrum (Tanacetum cinerariifolium) is caused by Stagonosporopsis tanaceti, with infected seed being a major means of transmission of this fungal pathogen. The infection process of S. tanaceti in pyrethrum seed and seedlings was determined. Infection hyphae only infected the outer and inner layers of the seed coat and not the embryo of naturally infected pyrethrum seed. During the process of germination of infected seed, S. tanaceti from the seed coat infected the developing embryo and cotyledon, resulting in pre‐ and post‐emergence death, depending on the level of infection in the seed coat. Pre‐emergence death occurred due to disintegration of the infected embryo, which was replaced by hyphae and extracellular anthocyanin‐like material (EAM) at 7 days after incubation (dai). Post‐emergence death occurred after both epidermal and cortical tissues of infected cotyledons at the crown/hypocotyl region disintegrated due to colonization by hyphae. Moreover, most of the tissues of the vascular bundles and cortical tissues contained heavy depositions of EAM at 10–14 dai. In 6‐week‐old infected seedlings, hyphae were confined to the epidermis and the cortical tissues at the crown/hypocotyl regions; the vascular bundles of both infected and uninfected regions, and cortical tissues of the uninfected regions of the seedlings were completely free from infection hyphae and EAM. These findings provide a better understanding of the early stages of the disease cycle of S. tanaceti and will lead to improved control measures for seedborne infection using seed treatments.     

TaqMan PCR assay for detection and quantification of <Emphasis Type="Italic">Stagonosporopsis tanaceti</Emphasis> in pyrethrum seed and seedlings

Pyrethrum seed has an important role in the transmission of Stagonosporopsis tanaceti, the cause of ray blight disease of pyrethrum. A TaqMan probe based polymerase chain reaction (PCR) assay was developed to quantify the level of S. tanaceti inocula in pyrethrum seed and seedlings. Primer pair (St_qF3, St_qR2) was designed based on the intergenic spacer (IGS) region of S. tanaceti, which produced a 125 bp amplicon specific to S. tanaceti. TaqMan PCR assay using St_qF3, St_qR2 and a probe St_qP was highly specific against the genomic DNA of S. tanaceti, but did not amplify DNA of 14 related Stagonosporopsis species or other foliar pathogens of pyrethrum. The sensitivity limit of this assay was measured using the cycle threshold (C_t) value which ranged from 17.59 for 10 nanograms (ng) to 36.34 for 100 femtograms (fg) genomic DNA of S. tanaceti. There was a significant negative correlation (r = ?0.999, P < 0.001) between the C_t value and the percent of S. tanaceti infected seed. In addition, this TaqMan PCR assay detected latent infection within seedlings. This assay could be applied to test commercial seed and seedlings before deciding on the appropriate management practices.     

A new anthracnose disease of pyrethrum caused by Colletotrichum tanaceti sp. nov

A new pathogen of pyrethrum (Tanacetum cinerariifolium) causing anthracnose was described as Colletotrichum tanaceti based on morphological characteristics and a four‐gene phylogeny consisting of rDNA‐ITS, β‐tubulin (TUB2), glyceraldehyde‐3‐phosphate dehydrogenase (GAPDH) and actin (ACT) gene sequences. The fungus produced perithecia in culture, requiring an opposite mating type isolate in a heterothallic manner. The initial infection strategy on pyrethrum leaves involved the formation of appressoria followed by production of multilobed infection vesicles in the epidermal cells. Infection and colonization then proceeded through thinner secondary hyphae, which resulted in the initial production of water‐soaked lesions followed by black necrotic lesions. The infection process was suggestive of a hemibiotrophic infection strategy. Moreover, phylogenetic analysis clearly showed that C. destructivum, C. higginsianum and C. panacicola were separate species that also had similar intracellular hemibiotrophic infection strategies as C. tanaceti, which all clustered in the C. destructivum complex. Colletotrichum spp. were detected at 1% incidence in seed of 1 of 19 seed lines, indicating the potential for seed as a source of inoculum into crops. Colletotrichum tanaceti was detected in leaf lesions from 11 of 24 pyrethrum fields surveyed between April and July 2012, at a frequency of 1·3–25·0% of lesions. Anthracnose probably contributes to the complex of foliar diseases reducing green leaf area in pyrethrum fields in Australia.     

Tan spot of pyrethrum is caused by a Didymella species complex

Tan spot is a disease of pyrethrum (Tanacetum cinerariifolium) in Australia. Recent increases in the severity and incidence of the disease have prompted a re‐evaluation of the pathogen, originally described as Microsphaeropsis tanaceti, including its phylogenetic relationships and morphology. Nucleotide comparison of partial sequences of the nuclear ribosomal internal transcribed spacer, β‐tubulin, large subunit 28S nrDNA (LSU), actin and glyceraldehyde‐3‐phosphate dehydrogenase loci identified two distinct haplotypes within the species. Haplotype differentiation was consistent for each locus, except for the LSU, within which sequences were identical across all isolates. Morphological variation, especially culture pigmentation and conidial size, consistently supported the phylogenetic data distinguishing two haplotypes. Phylogenetic comparisons of M. tanaceti incorporating 98 Didymellaceae species did not associate the M. tanaceti haplotypes with the genus Microsphaeropsis. The two M. tanaceti haplotypes were closely related, and clustered in the Didymella sensu stricto clade. Based on these phylogenetic results, supported by their distinct morphology and cultural characteristics, the two haplotypes of M. tanaceti are reclassified as two species of Didymella, namely D. rosea and D. tanaceti. The implications of two closely related species causing tan spot of pyrethrum are discussed.     

Tan spot: a new disease of pyrethrum caused by Microsphaeropsis tanaceti sp. nov.

The isolation frequency of Microsphaeropsis sp. in spring in association with necrotic lesions on leaves in Tasmanian pyrethrum (Tanacetum cinerariifolium) fields has increased substantially since first identification in 2001. Examination of morphological features and sequencing of the internal transcribed spacer region (ITS) resulted in the identification of a new species, herein described as Microsphaeropsis tanaceti sp. nov. The pathogenicity of three M. tanaceti isolates to two pyrethrum cultivars was confirmed by inoculating glasshouse‐grown plants in three experiments. No significant differences in the susceptibility of the two cultivars to infection by M. tanaceti were found. Symptoms were tan‐coloured spots which coalesced around the margins of the leaves. Therefore, the name ‘tan spot’ is proposed for this new disease of pyrethrum. The sensitivity of seven M. tanaceti isolates to difenoconazole and azoxystrobin, commonly used fungicides for the management of foliar diseases in spring, was assessed under in vitro conditions. Sensitivity testing for difenoconazole was conducted using a mycelial growth assay on potato dextrose agar, whilst testing for sensitivity to azoxystrobin used a conidial germination assay on water agar. Microsphaeropsis tanaceti was found to be more sensitive to azoxystrobin than difenoconazole, with complete inhibition of conidial germination at concentrations above 0·625 µg a.i. mL^?1. By comparison, concentrations of 50 µg a.i. difenoconazole mL^?1 or greater were required for significant inhibition of mycelial growth. It therefore appears likely that there is currently some control of tan spot as a result of the use of azoxystrobin and to a lesser extent, difenoconazole, for the control of other diseases.     

Epidemics of ray blight on pyrethrum are linked to seed contamination and overwintering inoculum of Phoma ligulicola var. inoxydabilis

Ray blight, caused by Phoma ligulicola var. inoxydabilis, is the most damaging disease of pyrethrum (Tanacetum cinerariifolium) in Australia. Data collected from 72 plots in commercial pyrethrum fields since 2001 to 2009 revealed substantial annual variations in isolation frequency of the pathogen during semidormancy of the crop in autumn and winter. Isolation frequency of the pathogen during this time and subsequent outbreaks of ray blight in spring were similar across the eight production regions where sampling was conducted, and isolation frequency of the pathogen was linearly correlated (r = 0.88; P < 0.0001) with subsequent defoliation severity when plants commenced growth in spring. Isolation frequency and defoliation severity also were correlated with the incidence of seed infested with P. ligulicola var. inoxydabilis (r = 0.71 and 0.44, respectively; P < 0.0001 in both correlations). Highly accurate risk algorithms for the occurrence of severe epidemics of ray blight were constructed using logistic regression. A model based solely on isolation frequency of the pathogen over autumn and winter correctly predicted epidemic development in 92% of fields. Another model utilizing the incidence of infested seed and rain-temperature interactions in early autumn (austral March and April) and late winter (austral June and July) had similar predictive ability (92% accuracy). Path analysis modeling was used to disentangle interrelationships among the explanatory variables used in the second logistic regression model. The analysis indicated that seedborne inoculum of P. ligulicola var. inoxydabilis contributes indirectly to ray blight defoliation severity through directly increasing overwintering frequency of the pathogen. Autumn and fall weather variables were modeled to have indirect effects on defoliation severity through increasing overwintering success of the pathogen but also direct effects on defoliation severity. Collectively, the analyses point to several critical stages in the disease cycle that can be targeted to minimize the probability of regional epidemics of ray blight in this perennial pathosystem.     

Late blight (Phytophthora infestans (Mont) De Bary) development from potato seed-pieces treated with fungicides

Fungicides were applied as seed-piece treatments to control potato late blight, caused by Phytophthora infestans, US8, A2 biotype in controlled environment and field experiments. Efficacy of seed treatments for controlling late blight was examined under three disease development regimes simulated by artifical inoculation; (a) seed-borne infection, (b) transmission of infection resulting from spread during the seed-cutting operation, and (c) infection of foliage by aerial inoculation. Emergence of plants from the seed-borne infection was uniformly low (<40%) in controlled environment and field experiments. In controlled environment experiments some of the plants that emerged from fungicide-treated seed-pieces were infected with late blight. Following exposure of tuber surfaces to P infestans, emergence rates from seed-pieces treated with formulated products that included mancozeb in the formulation were comparable to the untreated and non-inoculated control in controlled environment and field experiments. Plants that emerged from non-inoculated seed-pieces treated with fungicides that contained active ingredients known to be effective against foliar late blight had lower percentage foliar infection after inoculation than the untreated control. Leaves close to the base of the stem had fewer infections than leaves attached at the mid region of the main stem, 14 days after inoculation, in some of the controlled environment studies. In contrast, field experiments conducted under conditions conducive to late blight development showed that none of the seed treatments applied to late blight-free seed-pieces delayed the onset and severity of late blight infection. In potato production areas at risk of early season late blight, seed treatments applied to healthy seed may confer limited protection against late blight between planting and the first scheduled applications of prophylactic foliar fungicides. © 1999 Society of Chemical Industry     

Fungicide resistance in Cercospora species causing cercospora leaf blight and purple seed stain of soybean in Argentina

Cercospora species cause cercospora leaf blight (CLB) and purple seed stain (PSS) on soybean. Because there are few resistant soybean varieties available, CLB/PSS management relies heavily upon fungicide applications. Sensitivity of 62 Argentinian Cercospora isolates to demethylation inhibitor (DMI), methyl benzimidazole carbamate (MBC), quinone outside inhibitor (QoI), succinate dehydrogenase inhibitor (SDHI) fungicides, and mancozeb was determined in this study. All isolates were sensitive to difenoconazole, epoxiconazole, prothioconazole, tebuconazole, and cyproconazole (EC₅₀ values ranged from 0.006 to 2.4 µg/ml). In contrast, 51% of the tested isolates were sensitive (EC₅₀ values ranged from 0.003 to 0.2 µg/ml), and 49% were highly resistant (EC₅₀ > 100 µg/ml) to carbendazim. Interestingly, all isolates were completely resistant to azoxystrobin, trifloxystrobin, and pyraclostrobin, and insensitive to boscalid, fluxapyroxad, and pydiflumetofen (EC₅₀ > 100 µg/ml). The G143A mutation was detected in 82% (53) of the QoI-resistant isolates and the E198A mutation in 97% (31) of the carbendazim-resistant isolates. No apparent resistance mutations were detected in the succinate dehydrogenase genes (subunits sdhB, sdhC, and sdhD). Mancozeb completely inhibited mycelial growth of the isolates evaluated at a concentration of 100 µg/ml. All Argentinian Cercospora isolates were sensitive to the DMI fungicides tested, but we report for the first time resistance to QoI and MBC fungicides. Mechanism(s) other than fungicide target-site modification may be responsible for resistance of Cercospora to QoI and MBC fungicides. Moreover, based on our results and on the recent introduction of SDHI fungicides on soybean in Argentina, Cercospora species causing CLB/PSS are insensitive (naturally resistant) to SDHI fungicides. Insensitivity must be confirmed under field conditions.     

Use of survival analysis to assess management options for ray blight in Australian pyrethrum fields

The effects of fungicide, cultivar and plant density on the time‐to‐death of pyrethrum flowers affected by ray blight (caused by Phoma ligulicola var. inoxydablis) in Australia were analysed using nonparametric Kaplan–Meier (KM) estimates and accelerated failure time (AFT) models with a Weibull probability distribution. Analyses using KM estimates and AFT models yielded similar results. The median survival time (T) for all flowers in the fungicide trial was estimated at 53 days [95% confidence interval (CI) = 43–53] in 2000 and 60 days (CI = 51–60) in 2001. In both years, all fungicides tested except copper oxychloride significantly (P ≤ 0·0495) increased the duration of flower survival compared with nontreated plots. Significant variation (P < 0·0001) was noted between years and among four cultivars in terms of flower survival, with T values for different cultivars ranging from 41 to 81 days, and averaging 69 days (CI = 60–69) in 2005 and 64 days (CI = 56–64) in 2006 for all cultivars. Planting at a quarter the density currently recommended increased flower survival by 41·8% (χ² = 29·19; P < 0·0001), but did not increase yield. Linear regression indicated that defoliation severity accounted for at least 94% of variation in median survival time. Improved management may be achieved via an integrated strategy incorporating these factors.     

The novel elicitor COS‐OGA enhances potato resistance to late blight

Phytophthora infestans is the causal agent of potato late blight. This pathogen is usually controlled by fungicides, but new European regulations have imposed changes in crop protection management that have led to a search for alternative control measures. The induction of plant defence responses by elicitors is a promising new strategy compatible with sustainable agriculture. This study investigated the effect of eliciting a defence response in potato against P. infestans using a formulation of the COS‐OGA elicitor that combines cationic chitosan oligomers (COS) and anionic pectin oligomers (OGA). Trials were conducted under greenhouse conditions to assess the ability of COS‐OGA to control P. infestans. The results showed that three foliar applications with this elicitor significantly increased potato protection against late blight in controlled conditions. The activation of potato defence genes was also evaluated by RT‐qPCR during these trials. Two pathogenesis‐related proteins, basic PR‐1 and acidic PR‐2, were rapidly and significantly up‐regulated by the elicitor treatment. Therefore, these results suggest that the COS‐OGA elicitor increases the protection of potato against P. infestans and that this protection could be explained by an increase in the expression of potato defence genes rather than by biocide activity.     

Infection of alfalfa pollen bySclerotinia sclerotiorum

Blossom blight, caused bySclerotinia sclerotiorum, has become an important disease of alfalfa (Medicago sativa L.) in seed production areas of western Canada. Studies using light microscopy and scanning and transmission electron microscopy revealed that pollen grains of alfalfa are susceptible to infection byS. sclerotiorum. Ascospores ofS. sclerotiorum germinated readily in water with or without pollen grains. Examinations of ascospore—pollen mixtures incubated at room temperature (20–22°C) for 5 days revealed that numerous pollen grains were infected byS. sclerotiorum by direct hyphal penetration through the equatorial germinative pores or through the exine and intine layers of the pollen wall without the formation of infection cushions or appressoria. After penetration, hyphae ramified within the pollen grains, causing plasmolysis of the cytoplasmic membrane and eventual disintegration of the pollen cytoplasm. The study suggests that alfalfa pollen may play a role in the epidemiology of blossom blight in alfalfa.     

Effect of phosphate‐based seed priming on strigolactone production and Striga hermonthica infection in cereals

Strigolactones, plant‐secreted underground signalling molecules, play an important role in agricultural ecosystems, because they mediate the interaction of crops with symbiotic AM fungi and parasitic weeds like Striga hermonthica. Cereal host plants secret these signalling molecules particularly under nutrient‐deficient conditions and especially when phosphate (P) is limiting. The objective of the present study was to see the potential of P seed priming for S. hermonthica management in cereals in relation to strigolactone production. It has been demonstrated that P fertiliser application down‐regulates the production of these signalling molecules in the rhizosphere, which results in lower S. hermonthica infection of cereals. The laboratory study showed maximum production of strigolactones from dry and water‐soaked seeds, while seed soaking in P solution reduced their production. Similarly, maximum S. hermonthica infection was observed under control treatments with dry sowing or water soaking, while P seed soaking decreased S. hermonthica germination, emergence and dry biomass in all cereal crops. Our study shows that P seed priming resulted in lower exudation of strigolactones, which induced less S. hermonthica seeds germination and hence may lead to lower S. hermonthica infection. P‐based seed priming could prove to be an effective and affordable strategy to reduce S. hermonthica infection in cereals. Further research for practical field application is needed.     

Characterization of tuber blight‐suppressive soils from four provinces of the Ecuadorean Andes

Late blight, caused by the oomycete Phytophthora infestans, is a threat to potato‐cropping systems worldwide. In the Ecuadorian Andes, despite a high late blight incidence in foliage, tuber blight is rare. In this work, the hypothesis that Ecuadorian Andean soils are naturally suppressive to P. infestans tuber infection was evaluated. Soils from four potato‐growing regions were assessed for disease suppressiveness by determining the effects of soil heat treatment on P. infestans sporangia and their ability to infect potato slices after 1, 8, 15 and 30 days of exposure to soils. Tuber infection after inoculation with P. infestans‐infested soils was consistently lower during the evaluation period compared with heat‐treated soils. Fresh, untreated soils affected germination and viability of P. infestans sporangia in a site‐dependent manner. In addition, the effect of heat treatment on soil bacterial communities was assessed through terminal restriction fragment length polymorphism analysis of the 16S rDNA gene region. Heat treatment disrupted bacterial community composition, and a subset of terminal restriction fragments (TRF) was either positively or negatively correlated with tuber infection. Bacterial TRF negatively correlated with tuber infection corresponded in fragment size to taxa with known ability to inhibit pathogens and promote plant growth. Finally, bacterial isolates obtained from untreated soils, which inhibited P. infestans growth in vitro, represented 22–47% of isolates recovered, and matched classes predicted by the TRFs. This work represents a first step in understanding the mechanisms behind the low incidence of tuber blight in Andean potato‐cropping systems.     

Histological characterization of the early-stage infection events of Setosphaeria turcica in maize

Northern corn leaf blight (NCLB) caused by Setosphaeria turcica is a major foliar disease of maize. The early-stage infection events of this pathogen on maize leaves are unclear. We investigated the optimum temperature for conidial germination and appressorium formation, and characterized penetration and growth of S. turcica in maize leaf sheath and onion epidermis cells, including use of histological staining to assess plant cell viability. The results showed that the optimum temperature for conidial germination and appressorium formation was 20°C. On the maize leaf sheath, the appressoria were formed by germinated conidia, and penetration on the epidermal cells occurred at 8 h postinoculation (hpi). Round vesicles developed beneath the appressoria. Between 16 and 24 hpi, the branched invasive hyphae invaded three to five adjacent cells at most infection sites. The invasive hyphae tended to move along the cell wall and crossed from one cell to another. In the onion epidermis cells, the appressoria formed at 8 hpi, and in most cases the epidermal cells were penetrated through the juncture of the cell walls. At 16–24 hpi, the primary hyphal terminus swelled to a vesicle. The maize leaf sheath cells died at 8 hpi, whereas the onion cells did not. Our findings documented in detail the penetration and invasive hyphal growth in maize leaf sheath and onion epidermis, as well as viability of plant cells, at the early stages of infection, and provide a foundation for elucidating the underlying mechanism of S. turcica–maize interactions.     

Dynamics of the ascospore dispersal of Stagonosporopsis citrulli,a causal agent of gummy stem blight of cucurbits

Sexually reproduced, airborne ascospores of Stagonosporopsis citrulli may play a role in its dispersal. S. citrulli causes gummy stem blight (GSB), one of the most important foliar diseases of cucurbits. Four studies were conducted with S. citrulli to investigate for how long ascospores are released and how far they can be dispersed from a source field. In the first study, colonized watermelon debris was sampled during three seasons and samples were assayed for ascospore release. Ascospores were detected 292, 313, and 306 days after inoculation of the source. In the second study, the active release of ascospores from pseudothecia in a Petri dish was monitored for 7 days. The release of ascospores decreased by ≤90% from 1 day after the start of the assay until 7 days after. In the third study, trap plant assays were conducted to measure the dispersal gradient of ascospores up to 366 m from the source. Generally, frequency of pathogen recovery from trap plants decreased with increasing distance from the source. The ascospore dispersal data fitted the exponential model better than the power law model. In the final study, dispersal experiments were conducted under controlled conditions. The incidence of GSB decreased with increasing distance, up to 55 m, from the source. It was concluded that ascospores of S. citrulli can serve as primary inoculum for epidemics and could easily spread among fields. Debris from cucurbit crops can be the source of ascospores for up to 10 months and should be cleared expeditiously.     

Infection of mungbean seed by Macrophomina phaseolina is more likely to result from localized pod infection than from systemic plant infection

The ubiquitous fungal pathogen Macrophomina phaseolina is best known as causing charcoal rot and premature death when host plants are subject to post‐flowering stress. Overseas reports of M. phaseolina causing a rapid rot during the sprouting of Australian mungbean seed resulted in an investigation of the possible modes of infection of seed. Isolations from serial portions of 10 mungbean plants naturally infected with the pathogen revealed that on most plants there were discrete portions of infected tissue separated by apparently healthy tissue. The results from these studies, together with molecular analysis of isolates collected from infected tissue on two of the plants, suggested that aerial infection of aboveground parts by different isolates is common. Inoculations of roots and aboveground parts of mungbean plants at nine temperature × soil moisture incubation combinations and of detached green pods strongly supported the concept that seed infection results from infection of pods by microsclerotia, rather than from hyphae growing systemically through the plant after root or stem infection. This proposal is reinforced by anecdotal evidence that high levels of seed infection are common when rainfall occurs during pod fill, and by the isolation of M. phaseolina from soil peds collected on pods of mungbean plants in the field. However, other experiments showed that when inoculum was placed within 130 mm of a green developing pod and a herbicide containing paraquat and diquat was sprayed on the inoculated plants, M. phaseolina was capable of some systemic growth from vegetative tissue into the pods and seeds.     

Effect of Zinc Oxide Nanoparticles and Rhizobium leguminosarum on Growth,Photosynthetic Pigments and Blight Disease Complex of Pea

Effects of zinc oxide nanoparticles (ZnO NPs) and Rhizobium leguminosarum alone and in combination were observed on the disease complex of pea caused by Meloidogyne incognita and Pseudomonas syringae pv. pisi. Plants inoculated with M. incognita and P. syringae pv. pisi, alone or in combination, showed a significant reduction in plant growth, chlorophyll and carotenoid content compared to uninoculated controls. Use of ZnO NPs (0.10?ml^?1) as seed priming resulted in a greater increase in plant growth than 0.10?ml^?1 foliar spray. Plants inoculated with R. leguminosarum had better plant growth, chlorophyll and carotenoid content than plants without R. leguminosarum. Greater plant growth, chlorophyll and carotenoid content were observed when NPs primed seeds were grown with R. leguminosarum than the use of NPs foliar spray plus R. leguminosarum. Plants inoculated with R. leguminosarum showed higher root nodulation while only few nodules were observed in plants without R. leguminosarum. Both tested pathogens had adverse effect on nodulation, while use of ZnO NPs with R. leguminosarum also reduced nodulation. ZnO NPs and R. leguminosarum reduced blight disease indices, galling and nematode population. Use of ZnO NPs primed seeds with R. leguminosarum resulted in the highest reduction in disease indices, galling and nematode population. The segregation of various treatments in the biplot of principal component analysis demonstrates a suppressive role of ZnO NPs on blight disease complex of pea.

     

Activation of defence responses to Phytophthora infestans in potato by BABA

Late blight caused by Phytophthora infestans is one of the most devastating diseases of the potato crop. Resistance breeding and current fungicides are unable to control the rapidly evolving P. infestans and new control strategies are urgently needed. This study examined mechanisms of dl ‐β‐aminobutyric acid (BABA)‐induced resistance (IR) in the potato–P. infestans system. Leaves from two cultivars that differ in their degree of resistance, Bintje and Ovatio, were analysed after foliar treatment with BABA. Rapid activation of various defence responses and a significant reduction in P. infestans growth were observed in leaves treated with BABA. In the more resistant cultivar, Ovatio, the activation was both faster and stronger than in Bintje. Microscopic analysis of leaves treated with BABA revealed induction of small hypersensitive response (HR)‐like lesions surrounded by callose, as well as production of hydrogen peroxide (H₂O₂). Molecular and chemical analyses revealed soluble phenols such as arbutin and chlorogenic acid and activation of PR‐1. These results show a direct activation of defence responses in potato, rather than priming as reported for other plant species. They also show that the efficiency of BABA‐IR differs between cultivars, which highlights the importance of taking all aspects into consideration when establishing new methods for disease management.     

Paraphoma chlamydocopiosa sp. nov. and Paraphoma pye sp. nov., two new species associated with leaf and crown infection of pyrethrum

Two new pathogens of pyrethrum, described as Paraphoma chlamydocopiosa and Paraphoma pye, isolated from necrotic leaf lesions on pyrethrum plants in northern Tasmania, Australia, were identified using morphological characters, phylogenetic analysis of the internal transcribed spacer (ITS), elongation factor 1‐α (EF1‐α) and β‐tubulin (TUB) genes, and pathogenicity bioassays. Bootstrap support in the combined and individual gene region phylogenetic trees supported the two species that were significantly different from the closely related P. chrysanthemicola and P. vinacea. Morphological characteristics also supported the two new species, with conidia of P. chlamydocopiosa being considerably longer and wider than either P. chrysanthemicola or P. vinacea, and P. pye being distinct in forming bilocular pycnidia. Glasshouse pathogenicity tests based on root dip inoculation resulted in P. chlamydocopiosa and P. pye infecting the crown and upper root tissues of pyrethrum plants, and significant reduction in biomass 2 months after inoculation. Both of these Paraphoma species caused leaf lesions during in vitro and in vivo bioassays 2 weeks after foliar spray inoculation. Although P. chlamydocopiosa and P. pye were shown to be crown rot pathogens, they were also commonly isolated from leaves of diseased plants in pyrethrum fields of northern Tasmania.     

Hot-water baths,biologicals and re-curing effects on rhizopus soft rot during sweetpotato packing

Rhizopus soft rot (RSR) caused by Rhizopus stolonifer is one of the most devastating postharvest diseases of sweetpotato. It causes greatest losses when sweetpotatoes are removed from storage, washed and packed for marketing. The disease has been managed effectively by prophylactic application of synthetic fungicides on the packing line. However, there is increasing demand for alternative management strategies that do not rely on prophylactic use of synthetic fungicides. While curing immediately after harvest is a standard industry practice, re-curing after storage is not widely practised for postharvest disease management. In this study, the use of hot-water baths, biocontrol agents and re-curing after storage were investigated as potential replacements for synthetic fungicides that are widely used during sweetpotato packing. Hot-water baths at 52 °C for 10–15 min immediately after inoculation reduced RSR incidence by as much as 75%, but increased susceptibility to post-treatment Rhizopus infection. The biological control product Bio-Save® (a.i. Pseudomonas syringae strain ESC-10), used in conjunction with a 4 min water bath at 52 °C, gave similar protection (1.2% RSR, t = −1.7, P = 0.514) as the industry standard treatment with dicloran. Re-curing for as little as 4 h after washing roots significantly reduced RSR and deserves further evaluation to optimize conditions and determine its influence on other postharvest diseases.