Comparison of mineral oil, insecticidal and biopesticide spraying regimes for reducing spread of three potato virus Y (PVY) strains in potato crops

In-field management of potato virus Y (PVY) faces challenges by changing availability and environmental acceptability of chemical agents to control aphid vectors of the virus, and by proliferation of PVY strains with different symptomology and rates of spread. Over 2018-2020, foliar spray treatments were compared in field experiments in New Brunswick, Canada, to measure effectiveness at reducing spread of PVYO, PVYN:O and PVYNTN strains. Mineral oil, insecticide, combined oil and insecticide spray and a biopesticide (i.e., LifeGard® WG) were compared. Insecticide-only and mineral oil-only were not effective, though several combined oil and insecticide, and biopesticide treatments significantly reduced PVY spread. The biopesticide was proportionately more effective with recombinant PVYN:O and PVYNTN strains, possibly from exciting the plant’s hypersensitive resistance response only caused naturally in cv. Goldrush by PVYO. Pesticide residue analysis showed that mineral oil enhanced the retention of pyrethroid insecticide in the potato foliage longer than with insecticide applied alone, which may explain the beneficial synergistic effect of combined sprays for reducing PVY spread. Tuber yields were generally unchanged in chemical insecticide treatments but were slightly lower in biopesticide treatment. The cost per PVY protection was competitive across all effective treatments, including biopesticide, however, there was some revenue loss from lower yield with the biopesticide. This biopesticide is organic-certified, however, thus a small premium on price for organic production could offset this yield deficit.

Potato virus Y biological strain group YD: hypersensitive resistance genes elicited and phylogenetic placement

Potato virus Y (PVY) disrupts healthy seed potato production and causes tuber yield and quality losses globally. Its subdivisions consist of strain groups defined by potato hypersensitive resistance (HR) genes and phylogroups defined by sequencing. When PVY isolate PP was inoculated to potato cultivar differentials with HR genes, the HR phenotype pattern obtained resembled that caused by strain group PVYD isolate KIP1. A complete genome of isolate PP was obtained by high throughput sequencing. After removal of its short terminal recombinant segment, it was subjected to phylogenetic analysis together with 30 complete non-recombinant PVY genomes. It fitted within the same minor phylogroup PVYO3 sub-clade as KIP1. Putative HR gene Nd was proposed previously to explain the unique HR phenotype pattern that developed when differentials were inoculated with PVYD. However, an alternative explanation was that PVYD elicits HR, with HR genes Nc and Ny instead. To establish which gene(s) it elicits, isolates KIP1 and PP were inoculated to F1 potato seedlings from: (i) crossing Kipfler and White Rose with Ruby Lou, and (ii) self-pollinated Desiree and Ruby Lou; where Kipfler is susceptible (S) but White Rose, Desiree and Ruby Lou develop HR. With both isolates, the HR:S segregation ratios obtained fitted 5:1 for Kipfler x Ruby Lou, 11:1 for White Rose x Ruby Lou, and 3:1 for Desiree. Those for Ruby Lou were 68:1 (isolate PP) and 52:0 (isolate KIP1). Since potato is tetraploid, these ratios suggest PVYD elicits HR with Ny from Ruby Lou (duplex condition) and Desiree (simplex condition), and Nc from White Rose (simplex condition), but provide no evidence that Nd exists. Therefore, our differential cultivar inoculations and inheritance studies highlight that PVYD isolates elicit an HR phenotype in potato cultivars with either of two HR genes, Nc or Ny, so putative gene Nd can be discounted. Moreover, phylogenetic analysis placed the complete genome of isolate PP within the same minor phylogroup PVYO3 sub-clade as KIP1 which constitutes the most basal divergence within overall major phylogroup PVYO.

Natural genetic diversity in the potato resistance gene RB confers suppression avoidance from Phytophthora effector IPI-O4

RB is a potato gene that provides resistance to a broad spectrum of genotypes of the late blight pathogen Phytophthora infestans. RB belongs to the CC-NB-LRR (coiled-coil, nucleotide-binding, leucine-rich repeat) class of resistance (R) genes, a major component of the plant immune system. The RB protein detects the presence of Class I and II IPI-O effectors from P. infestans to initiate a hypersensitive resistance response, but this activity is suppressed in the presence of the Class III effector IPI-O4. Using natural genetic variation of RB within potato wild relatives, we identified two amino acids in the CC domain that alter interactions needed for suppression of resistance by IPI-O4. We have found that separate modification of these amino acids in RB can diminish or expand the resistance capability of this protein against P. infestans in both Nicotiana benthamiana and potato. Our results demonstrate that increased knowledge of the molecular mechanisms that determine resistance activation and R protein suppression by effectors can be utilized to tailor-engineer genes with the potential to provide increased durability.

Reactive oxygen species contribute to symptomless, extreme resistance to Potato virus X in tobacco

Here we show that in tobacco (Nicotiana tabacum cv. Samsun NN Rx1) the development of Rx1 gene-mediated, symptomless, extreme resistance to Potato virus X (PVX) is preceded by an early, intensive accumulation of the reactive oxygen species (ROS) superoxide (O2.-), evident between 1-6 hours after inoculation and associated with enhanced NADPH oxidase activities. This suggests a direct contribution of this ROS to virus restriction during symptomless, extreme resistance. Superoxide inhibition in PVX-inoculated leaves by infiltration of antioxidants (superoxide dismutase, SOD and catalase, CAT) partially suppresses extreme resistance parallel with the appearance of localized leaf necrosis resembling a hypersensitive resistance response (HR). F1 progeny from crosses of Rx1 and ferritin-overproducer (deficient in production of the ROS OH.) tobaccos also display a suppressed extreme resistance to PVX, since significantly increased virus levels are coupled to HR, suggesting a role of the hydroxyl radical (OH.) in this symptomless antiviral defense. In addition, treatment of PVX-susceptible tobacco with a superoxide-generating agent (riboflavin/methionine) results in HR-like symptoms and reduced PVX titers. Finally, by comparing defense responses during PVX-elicited symptomless, extreme resistance and HR-type resistance elicited by Tobacco mosaic virus (TMV) we conclude that defense reactions typical of an HR (e.g. induction of cell death/ROS-regulator genes and antioxidants) are early and transient in the course of extreme resistance. Our results demonstrate the contribution of early accumulation of ROS (superoxide, OH.) in limiting PVX replication during symptomless extreme resistance and support earlier findings that virus-elicited HR represents a delayed, slower resistance response than symptomless, extreme resistance.

Molecular and biological characterization of recombinant isolates of Potato virus Y circulating in potato fields in Mexico

Potato virus Y (PVY) is a significant threat to potato production in Mexico. The presence of recombinant strains of PVY circulating in potato has been reported in the country, but no systematic study on the genetic diversity of PVY in potato and prevalence of PVY strains has been conducted yet. Here, we report on a series of surveys in seed potato production areas in two states in Mexico, Chihuahua and Jalisco, between 2011 and 2019. PVY was detected through the period of nine years in multiple potato cultivars in both states, often remaining asymptomatic in the most popular cultivars, such as Fianna and Agata. When typed to strain, all PVY samples studied were found to have N-serotype, and were all identified molecularly as isolates of the same recombinant strain, PVYNTN. Five of these PVY isolates were tested on tobacco, where they induced vein necrosis supporting the molecular typing. This identification was also confirmed biologically on differential potato cultivars, where one PVYNTN isolate from the 2013 survey triggered the hypersensitive resistance conferred by the Nztbr gene in the cv. Maris Bard. Seven of these Mexican PVYNTN isolates, collected between 2013 and 2019, including two PVY isolates from potato tubers exhibiting potato tuber necrotic ringspot disease, were subjected to whole genome sequencing and found to show a typical PVYNTNa recombinant structure. When subjected to phylogenetic analysis, Mexican PVYNTN sequences clustered in more than three separate clades, suggesting multiple introductions of PVYNTN in the country. The wide circulation of the PVYNTN strain in Mexican potato should be taken into account by the potato producers, to develop mitigation strategies for this PVY strain associated with tuber necrotic symptoms.

Natural genetic diversity in the potato resistance gene RB confers suppression avoidance from Phytophthora effector IPI-O4

ABSTRACTRB is a potato gene that provides resistance to a broad spectrum of genotypes of the late blight pathogen Phytophthora infestans. RB belongs to the CC-NB-LRR (coiled-coil, nucleotide-binding, leucine-rich repeat) class of resistance (R) genes, a major component of the plant immune system. It directly interacts with Class I and II IPI-O effectors from P. infestans to initiate a hypersensitive resistance response, but this activity is suppressed in the presence of the Class III effector IPI-O4. Using natural genetic variation of RB within potato wild relatives, we identified two amino acids in the CC domain that alter interactions needed for suppression of resistance by IPI-O4. We have found that separate modification of these amino acids in RB can diminish or enhance the resistance capability of this protein against P. infestans in both Nicotiana benthamiana and potato. Our results demonstrate that increased knowledge of the molecular mechanisms that determine resistance activation and R protein suppression by effectors can be utilized to tailor-engineer genes with the potential to provide increased durability.

HCPro Suppression of Callose Deposition Contributes to Strain-Specific Resistance Against Potato Virus Y

Potato virus Y (PVY; Potyviridae) is a continuing challenge for potato production owing to the increasing popularity of strain-specific resistant cultivars. Hypersensitive resistance (HR) is one type of plant defense responses to restrict virus spread. In many potato cultivars, such as cultivar Premier Russet (PR), local necrosis at the site of infection protects against the most common PVYO strain, but the HR often fails to restrain necrotic strains, which spread systemically. Here, we established the role of callose accumulation in the strain-specific resistance responses to PVY infection. We first uncovered that PVY, independent of the strain, is naturally capable of suppressing pathogenesis-related callose formation in a susceptible host. Such activity can be dissociated from viral replication by the transient expression of the viral-encoded helper component proteinase (HCPro) protein, identifying it as the pathogen elicitor. However, unlike the necrotic strain, PVYO and its corresponding HCPro are unable to block callose accumulation in resistant PR potatoes, in which we observed an abundance of callose deposition and the inability of the virus to spread. The substitution of eight amino acid residues within the HCPro C-terminal region that differ between PVYO and PVYN strains and were previously shown to be responsible for eliciting the HR response, are sufficient to restore the ability of HCProO to suppress callose accumulation, despite the resistant host background, in line with a new viral function in pathogenicity.

Progress in Developing Bacterial Spot Resistance in Tomato

Bacterial spot (BS), caused by four species of Xanthomonas: X. euvesicatoria, X. vesicatoria, X. perforans and X. gardneri in tomato (Solanum lycopersicum L.) results in severe loss in yield and quality by defoliation and the appearance of lesions on fruits, respectively. The combined industry standard for BS control (foliar applications Actigard® rotated with copper plus mancozeb) does not offer sufficient protection, especially when weather conditions favor disease spread. Development of tomato cultivars with BS resistance is thus an important measure to minimize losses. Hypersensitive and non-hypersensitive resistance has been identified in different wild accessions and cultivated tomato relatives and has been transferred to cultivated tomato. However, complete resistance is yet to be obtained. With the advent of next generation sequencing and precise genome editing tools, the genetic regions that confer resistance to bacterial spot can be targeted and enriched through gene pyramiding in a new commercial cultivar which may confer higher degree of horizontal resistance to multiple strains of Xanthomonas causing bacterial spot in tomato.

Reaction of Nicotiana species and cultivars of tobacco to Tobacco mosaic virus and detection of the N gene that confers hypersensitive resistance

Tobacco mosaic virus (TMV) brings increasing losses in the cultivation of tobacco. Sixty-two cultivars of tobacco and eleven species of Nicotiana were evaluated for resistance to TMV. Biological tests at two temperature ranges, DAS-ELISA and molecular markers were applied to assess the resistance to TMV. Most cultivars of tobacco showed susceptibility (S) to TMV, two were tolerant (T), while others revealed a hypersensitive response (HR). Hypersensitivity, determined by the N gene, occurred only at a temperature below 22°C. At a temperature above 28°C, all the cultivars showed mosaic discolorations or extensive necrosis. The reaction of the Nicotiana species was dependent on growth conditions. At 22°C, the reactions of sensitivity, tolerance and hypersensitivity to TMV were all observed, whereas above 28°C the species showed systemic necrotic symptoms. N. gossei was an exception because hypersensitivity occurred regardless of the thermal conditions. The resistance of this species was not conditioned by the N gene, which suggests that N. gossei could be an additional genetic resource for tobacco breeding.