Late blight caused by Phytophthora infestans is a serious threat to potato production. Currently, solutions to manage late blight in organic systems are scarce. This study was undertaken to evaluate the effect of selenium (Se) on plant emergence from seed pre-treated with Se and the seed decay pathogens Pectobacterium carotovorum ssp. carotovorum and Fusarium sambucinum, and on foliar potato late blight following inoculation with P. infestans and foliar treatment with Se. The potential modes of action of Se were also evaluated. Following inoculation of the seed decay pathogens on potato seed pre-treated with different rates of Se, plant emergence was not affected by Se treatment. By pre-treating potato seed with different Se rates, and applying different foliar Se rates on potato plants subsequently inoculated with the late blight pathogen in a greenhouse setting, we showed that foliar Se application alone or combined with Se seed treatment reduced the severity and incidence of late blight, and activated the production of secondary metabolites in potato leaves and tubers. Further, in vitro studies showed that Se directly inhibited the mycelial growth of P. infestans and other important plant pathogens, including Fusarium graminearum and Sclerotinia sclerotiorum. The data suggest that Se acts as an inducer of plant defences, while also inhibiting fungal growth. The study highlights the potential of Se for managing late blight, and suggests that its application could contribute to improving plant health in greenhouse production systems by helping to control diseases caused by P. infestans, F. graminearum and S. sclerotiorum.
Single applications of liquid swine manure (LSM; ca. 49 hL/ha), ammonium lignosulfonate (ALS; a product derived from the pulp and paper processing industry, 50% solids at ca. 10 hL/ha), and Nature Safe (NS) 10:2:8 (N:P:K; an organic fertilizer made primarily of poultry feathers, ca. 8.6 t/ha) were made at two commercial potato (Solanum tuberosum) fields (sites P-I and P-II) in Prince Edward Island in the spring of 1999. Potato tubers were planted 3 weeks later and again in 2000 and 2001 without further addition of amendments. The effects on potato scab, black scurf, silver scurf, fusarium dry rot, verticillium wilt, and tuber yield were determined. Potato scab severity was significantly reduced (77%–84%) by NS compared with the controls at both sites in the first year. Scab severity remained significantly lower in the second year, but there was no difference from the controls in the third year. ALS significantly reduced scab severity (54%) compared with the controls at both sites in the first year, and scab severity remained lower than the control at only one site (P-I) and only for the second year. The mechanism by which ALS reduced scab severity is not known. LSM significantly reduced scab severity (38%) at one site only (P-II) and only in the first year. The control at site P-II was likely due to the favourable pH (5.4) for biotoxic activity of volatile fatty acids (VFAs) in the LSM, whereas the soil pH of 6.4 at site P-I was unfavourable for VFA toxicity to Streptomyces scabiei. The severity of black scurf was significantly reduced compared with the control at both sites by LSM (51% at P-I and 48% at P-II) and NS (40% at P-I and 61% at P-II) but in the first year only. There was no significant effect on the severity of silver scurf or fusarium dry rot or tuber yield by any amendment in any year. Verticillium wilt was significantly reduced compared with the control by ALS (88%) and NS (66%) at only one site (P-I) and only in the second year. Marketable yield was increased at both sites for NS and ALS, and at one site for LSM.
The culturable component of bacterial communities found in the endoroot and associated exoroot (root zone soil) was examined in potatoes (Solanum tuberosum L.) grown under either conventional or minimum tillage systems. Bacterial species – abundance relationships were determined and in vitro antibiosis ability investigated to discover whether tillage practice or bacteria source (endo- or exoroot) influenced bacterial community structure and functional versatility. Antibiosis abilities against Phytophthora erythroseptica Pethyb. (causal agent of pink rot of potatoes), Streptomyces scabies (Thaxt.) Waksm. and Henrici) (causal agent of potato common scab), and Fusarium oxysporum Schlecht. Emend. Snyder and Hansen (causal agent of fusarium potato wilt) were selected as indicators of functional versatility. Bacterial community species richness and diversity indices were significantly greater (P = 0.001) in the exoroot than in the endoroot. While both endo- and exoroot communities possessed antibiosis ability against the phytopathogens tested, a significantly greater proportion (P = 0.0001) of the endoroot population demonstrated antibiosis ability than its exoroot counterpart against P. erythroseptica and F. oxysporum. Tillage regime had no significant influence on species-abundance relationships in the endo- or exoroot but did influence the relative antibiosis ability of bacteria in in vitro challenges against S. scabies, where bacteria sourced from minimum tillage systems were more likely to have antibiosis ability (P = 0.0151). We postulate that the difference in the frequency of isolates with antibiosis ability among endoroot versus exoroot populations points to the adaptation of endophytic bacterial communities that favour plant host defence against pathogens that attack the host systemically.Key words: antibiosis, bacterial endophytes, Fusarium oxysporum, Phytophthora erythroseptica, Streptomyces scabies, potatoes.
Early blight, caused by Alternaria solani , is a ubiquitous disease that can reduce potato yield. Adequate crop fertility and appropriate fungicide applications usually suppress the development of this disease. Field trials were established in Prince Edward Island to determine whether strobilurin analogs, namely azoxystrobin and pyraclostrobin, could suppress early blight of potatoes (cvs. Shepody and Russet Burbank) grown under two nitrogen (N) fertility regimes (high or low N). Azoxystrobin and pyraclostrobin were linked to significantly higher total tuber yield for Russet Burbank in 2003 and Shepody in 2004 when compared to plots receiving no strobilurin fungicides. No significant differences in total tuber yield based on N fertility were observed, although the high N rate decreased disease in Russet Burbank control plots in 2004. Due to an absence of early blight in 2003, the increase in Russet Burbank yield may be attributed to the physiological and developmental alterations brought about by strobilurin products in treated plants. In 2004, early blight was severe in inoculated control plots, but it was significantly suppressed in plots having received azoxystrobin or pyraclostrobin, regardless of the cultivar type or fertility regime. Therefore, to prevent unnecessary N inputs, growers need to supply only the necessary N amount to optimize tuber yields and manage early blight with fungicides.
Late blight, caused by Phytophthora infestans, is a devastating disease of potatoes and tomatoes that causes significant crop losses worldwide. Late blight diversity has been increasing since 2009 in many areas of Canada on both potatoes and tomatoes. Although the US-8 genotype of P. infestans has dominated pathogen populations in much of Canada for the past 15 years, novel genotypes have emerged in recent years. A comprehensive survey of late blight samples collected across Canada in 2011 indicated that although the US-8 genotype of P. infestans still dominated pathogen populations in Prince Edward Island, new US-23 and US-24 genotypes had become established in other parts of Canada. The US-11 and US-22 genotypes dominated populations of the pathogen from specific areas of Alberta and Ontario, respectively. Clonal propagation and migration contributed to long-distance transport of P. infestans genotypes and in some situations, both A1 and A2 mating types of the pathogen were found in the same production region. This finding is of concern because it increases the potential for sexual recombination within pathogen populations, leading to the production of oospores that can survive in the absence of host tissue and create new strains of P. infestans. Evidence for recombination in the P. infestans population was detected in Ontario on tomato. Segregation of Gpi allozymes and mating type appeared to occur independently of the RG57 loci, producing several new P. infestans genotypes. Prediction and management of late blight will need to be evaluated and modified to accommodate these new potentially aggressive P. infestans genotypes.
