Tamilarasan Thangavel

Monitoring Spongospora subterranea Development in Potato Roots Reveals Distinct Infection Patterns and Enables Efficient Assessment of Disease Control Methods

Spongospora subterranea is responsible for significant potato root and tuber disease globally. Study of this obligate (non-culturable) pathogen that infects below-ground plant parts is technically difficult. The capacity to measure the dynamics and patterns of root infections can greatly assist in determining the efficacy of control treatments on disease progression. This study used qPCR and histological analysis in time-course experiments to measure temporal patterns of pathogen multiplication and disease development in potato (and tomato) roots and tubers. Effects of delayed initiation of infection and fungicidal seed tuber and soil treatments were assessed. This study found roots at all plant developmental ages were susceptible to infection but that delaying infection significantly reduced pathogen content and resultant disease at final harvest. The pathogen was first detected in roots 15–20 days after inoculation (DAI) and the presence of zoosporangia noted 15–45 DAI. Following initial infection pathogen content in roots increased at a similar rate regardless of plant age at inoculation. All fungicide treatments (except soil-applied mancozeb which had a variable response) suppressed pathogen multiplication and root and tuber disease. In contrast to delayed inoculation, the fungicide treatments slowed disease progress (rate) rather than delaying onset of infection. Trials under suboptimal temperatures for disease expression provided valuable data on root infection rate, demonstrating the robustness of monitoring root infection. These results provide an early measure of the efficacy of control treatments and indicate two possible patterns of disease suppression by either delayed initiation of infection which then proceeds at a similar rate or diminished epidemic rate.

Toughing It Out—Disease-Resistant Potato Mutants Have Enhanced Tuber Skin Defenses

Common scab, a globally important potato disease, is caused by infection of tubers with pathogenic Streptomyces spp. Previously, disease-resistant potato somaclones were obtained through cell selections against the pathogens toxin, known to be essential for disease. Further testing revealed that these clones had broad-spectrum resistance to diverse tuber-invading pathogens, and that resistance was restricted to tuber tissues. The mechanism of enhanced disease resistance was not known. Tuber periderm tissues from disease-resistant clones and their susceptible parent were examined histologically following challenge with the pathogen and its purified toxin. Relative expression of genes associated with tuber suberin biosynthesis and innate defense pathways within these tissues were also examined. The disease-resistant somaclones reacted to both pathogen and toxin by producing more phellem cell layers in the tuber periderm, and accumulating greater suberin polyphenols in these tissues. Furthermore, they had greater expression of genes associated with suberin biosynthesis. In contrast, signaling genes associated with innate defense responses were not differentially expressed between resistant and susceptible clones. The resistance phenotype is due to induction of increased periderm cell layers and suberization of the tuber periderm preventing infection. The somaclones provide a valuable resource for further examination of suberization responses and its genetic control.

Resistance to Multiple Tuber Diseases Expressed in Somaclonal Variants of the Potato Cultivar Russet Burbank

Multiple disease resistance is an aim of many plant breeding programs. Previously, novel somatic cell selection was used to generate potato variants of “Russet Burbank” with resistance to common scab caused by infection with an actinomycete pathogen. Coexpression of resistance to powdery scab caused by a protozoan pathogen was subsequently shown. This study sought to define whether this resistance was effective against additional potato tuber diseases, black scurf, and tuber soft rot induced by fungal and bacterial pathogens. Pot trials and in vitro assays with multiple pathogenic strains identified significant resistance to both tuber diseases across the potato variants examined; the best clone A380 showed 51% and 65% reductions in disease severity to tuber soft rot and black scurf, respectively, when compared with the parent line. The resistance appeared to be tuber specific as no enhanced resistance was recorded in stolons or stem material when challenged Rhizoctonia solani that induces stolon pruning and stem canker. The work presented here suggests that morphological characteristics associated with tuber resistance may be the predominant change that has resulted from the somaclonal cell selection process, potentially underpinning the demonstrated broad spectrum of resistance to tuber invading pathogens.

First Report of Systemic Downy Mildew of Opium Poppy Caused by Peronospora somniferi in Australia

© The American Phytopathological Society.Opium poppy (Papaver somniferum) is a highly valuable crop grown for its opiate alkaloid content, primarily morphine, thebaine, and codeine. Australia is the world’s largest producer of licit opiates, cultivating ∼50% of world poppy production, with an estimated farm gate value of

Somaclonal selection in potato for resistance to common scab provides concurrent resistance to powdery scab

60 million. Downy mildew has been a major constraint to poppy production since it was first recorded in 1996 (Cotterill and Pascoe 1998; Scott et al. 2004). The pathogen was recently reclassified as Peronospora meconopsidis (Voglmayr et al. 2014). This downy mildew species produces vein-delimited, angular, localized necrotic lesions with sparse sporulation on abaxial leaf surfaces. A second downy mildew species, P. somniferi, has been recorded to produce systemic infections, resulting in stunting and deformation of opium poppy in Europe (Voglmayr et al. 2014). In the 2013/14 season, stunted poppy plants with chlorotic distorted leaves, little or no necrosis, and profuse abaxial sporulation were observed within commercial crops in Tasmania, Australia, for the first time. Affected plants also had strongly distorted stems. These symptoms were characteristic of systemic downy mildew infection. In the 2014/15 season, the incidence of plants with systemic downy mildew symptoms was widespread and resulted in complete crop loss in some fields. Pathogen samples were collected from leaves with systemic symptoms. Microscopic examination revealed the presence of hyaline conidiophores and conidia typical of Peronospora. Conidiophores (n = 10) were 290 to 530 μm long, with bifurcating branches. Paired terminal branchlets (n = 24) were 4.2 to 15.9 μm long and straight to slightly curved. Ovoid, hyaline conidia (n = 24) were 17.8 to 23.8 μm long and 14.3 to 17.2 μm wide, with length/width ratios between 1.19 and 1.47. Leaves with systemic symptoms were collected from a commercial field at Gawler in December 2015. Conidial suspensions were collected from leaves by rinsing with sterile deionized water and used to inoculate 6- to 8-week-old poppy seedlings. Seedlings were maintained in a growth chamber (16 to 21°C, 10:14 h dark/light regime; relative humidity 70 to 90%). Systemic symptoms of distorted, chlorotic leaves and stunted plants were observed after 14 to 21 days. Disease development was acropetal, with symptomless lower leaves and obvious symptoms on upper leaves. DNA was extracted from infected leaf tissues and extracts tested by PCR using primers targeting conserved regions of the coxI (Robideau et al. 2011) and coxII (Hudspeth et al. 2000) genes in Peronospora spp. Amplicons of 702 and 603 bp were obtained for the coxI and coxII gene regions, respectively. These were sequenced (GenBank accession nos. KX242325 and KX242326) and compared with published sequences (Voglmayr et al. 2014). Maximum likelihood phylogenetic analysis using a concatenate alignment of these two gene regions confirmed that systemic symptoms on Tasmanian opium poppy crops were caused by P. somniferi. This is the first record of P. somniferi in Australia and represents a major new threat to poppy production in this country.