Anastomosis groups ofRhizoctonia solani on potato in central México and potential for biological and chemical control

Anastomosis group (AG) identity ofRhizoctonia solani isolates collected from potato plants growing near Leon, Guanajuato, in central Mexico was determined. In samples from 15 fields, we found AG-3 and AG-4 with a frequency of 73.5% and 26.5% respectively. AG-4 was found only during the flowering stage of plant growth, whereas AG-3 was present on plants at every stage of development. The efficacy againstR. solani, in vitro andin situ, of certain biocontrol agents (Bacillus subtilis, Gliocladium virens) and fungicides (Pencycuron, Tolcoflos-methyl, Fluazinam, Azoxystrobin) was assessed. CertainBacillus strains collected from the field inhibitedin vitro growth of isolates of certain AGs. Only Pencycuron and Tolcoflos-methyl inhibited AG-3 100%in vitro, whereas in the field, Azoxystrobin and Pencycuron showed the highest efficacy against development of sclerotia on tubers.     

Diverse no-till irrigated crop rotations instead of burning and plowing continuous wheat

Field burning of residue is a traditional management tool for irrigated wheat (Triticum aestivum L.) production in the Inland Pacific Northwest of the United States (PNW) that can result in reduced air quality. A 6-year no-till field experiment to evaluate two complete cycles of a 3-year irrigated crop rotation of winter wheat–spring barley (Hordeum vulgare L.)–winter canola (Brassica napus L.) was sown (i) directly into standing residue of the previous crop, (ii) after mechanical removal of residue and, (iii) after burning of residue. The traditional practice of continuous annual winter wheat sown after burning residue and inverting the topsoil with a moldboard plow was included as a check treatment. Over-winter precipitation storage efficiency (PSE) was markedly improved when residue was not burned or burned and plowed after grain harvest. Grain yield of winter wheat trended higher in all no-till residue management treatments compared to the check treatment. Average grain yields of spring barley and canola were not significantly different among the no-till residue management treatments. Winter canola failed in 5 of 6 years due to a combination of a newly identified Rhizoctonia damping-off disease caused by Rhizoctonia solani AG-2-1 and cold temperatures that necessitated replanting to spring canola. Six-year average net returns over total costs were statistically equal over all four systems. All systems lost from $358 to $396 ha^?1. Soil organic carbon (SOC) increased linearly each year with no-till at the 0–5 cm depth and accumulated at a slower rate at the 5–10 cm depth. Take-all of wheat caused by Gaeumannomyces graminis var. tritici was most severe in continuous annual winter wheat. The incidence and severity of Rhizoctonia on roots of wheat and inoculum of R. solani AG-8, was highest in the no-till treatments, but there was no grain yield loss due to this disease in any treatment. Residue management method had no consistent effect on Rhizoctonia root rot on barley. The annual winter grass downy brome (Bromus tectorum L.) was problematic for winter wheat in the standing and mechanically removed residue treatments, but was controlled in the no-till residue burned and the burn and plow check. Another winter annual grass weed, rattail fescue (Vulpia myuros L.), infested all no-till treatments. This was the first comprehensive and multidisciplinary no-till irrigated crop rotation study conducted in the Pacific Northwest.     

Anastomosis Group and Pathogenicity of <Emphasis Type="Italic">Rhizoctonia solani </Emphasis>Associated with Stem Canker and Black Scurf of Potato in Heilongjiang Province of China

From 2012 to 2015, a total of 226 isolates of Rhizoctonia solani were collected from the stem cankers on potato stems and sclerotia on tubers from different potato cultivation areas of Heilongjiang Province, China. These isolates were assigned to the anastomosis group (AG) by performing conventional PCR assays using previously published primers for ITS-rDNA regions, as well as by observing hyphal interactions where appropriate. Most of the isolates were assigned to AG-3PT (58.85 %), and several were assigned to AG-5 (21.68 %), AG-2-1 (7.08 %) and AG-4 (12.39 %). Pathogenicity tests showed that the AG-3 and AG-5 isolates had the highest virulence, and the disease indices were 1.96a and 2.47a for stem and 1.48a and 1.6a for root (P < 0.05) after analyzed by LSD multiple comparisons, respectively. Both two isolates consistently caused large brown lesions with sunken on the potato stems and roots in in vitro and greenhouse experiments. This is the first detailed report on the AG composition, variability and pathogenicity of R. solani isolates associated with stem cankers and black scurf found on potatoes cultivated in Heilongjiang Province.     

Crop rotation and N fertilization effects on growth,yield, and disease incidence in potato

Crop rotation can be an effective mechanism for reducing disease incidence and contributing nitrogen (N) to succeeding crops. Interactions of plant pathogen suppression and soil nutrient availability may also exist, adding to the cropping system complexity. This study examined the impact of crop rotation, N fertilization, and their interaction on growth, yield, andRhizoctonia solani incidence in potato (Solanum tuberosum L. Norwis). Potato was grown continuously and in two-year rotations with annual alfalfa (Medicago sativa L. Nitro), hairy vetch (Vicia villosa Roth), white lupin (Lupinus albus L. Ultra), and oat (Avena sativa Astro). Fertilizer was banded at potato planting with 0, 45, 90, 135, 180, or 225 kg N ha^?1 as (NH₄)₂SO₄. Approximately 58% of continuous potato possessed stem lesions caused byR. solani, but only 12 to 22% of potato stems from other rotations possessed lesions. Tuber dry weight was affected by crop rotation in 1989, a dry year, but not in 1990. Apparent N fertilizer replacement values for hairy vetch, Nitro alfalfa, white lupin, and oat were 65, 43, 26, and 11 kg N ha^?1, respectively. All crop rotations studied appeared to enhance potato production by reducing stem infection byR. solani. Vetch and alfalfa provide additional benefits through their N contributions.     

Rhizoctonia disease of potatoes (Rhizoctonia solani): Fungicidal efficacy and cultivar susceptibility

Rhizoctonia stem canker and black scurf is an economically important disease of potatoes in Alberta and around the world. It reduces the quality and yield of potatoes and has become an important impediment for export of seed potatoes, especially to Mexico. Seed treatment using fungicides, presently registered in Canada, are not effective in controlling the disease to growers’ satisfaction. Field experiments conducted to evaluate the efficacy of various fungicides showed that captan (Captan), iprodione (Rovral), mancozeb (Tuberseal), and fludioxonil (Maxim) provide effective control of the disease. Of these fungicides, fludioxonil consistently provided superior protection. Based on the results of this study, Maxim was registered in 2000 for control of this disease in Canada. Potato cultivars/breeding selections showed a range of susceptibility reactions toR. solani but none of these was completely resistant to the disease. The information on comparative susceptibility of potato cultivars will help the growers make informed decisions regarding the management of this disease. Of six plant species (Beta vulgaris,Brassica compestris, Hordeum vulgare, Pisum sativum,Triticum aestivum, Zea mays) most commonly used in rotation with potatoes in Alberta, none was infected by potato isolates ofR. solani. The results suggested that use of these plant species as rotation crops with potatoes in Alberta will help in reducing the development of rhizoctonia disease in potatoes.     

Three Genes Related to Trehalose Metabolism Affect Sclerotial Development of Rhizoctonia solani AG-1 IA,Causal Agent of Rice Sheath Blight

Trehalose metabolism is related to the sclerotial development of Rhizoctonia solani AG-1 IA, the causal agent of rice sheath blight (RSB). Here, we further elucidated the functions of three genes Rstre, Rstps1 and Rstpp that encode three key enzymes trehalase (TRE), alpha, alpha-trehalose- phosphate synthase (TPS1) and trehalose 6-phosphate phosphatase (TPP) in the sclerotial development of R. solani AG-1 IA. Due to the lack of a stable genetic transformation system for R. solani, the heterologous expression of these three genes in Pichia pastoris GS115 was performed. The results showed that reactive oxygen species (ROS) contents and enzyme activities in R. solani decreased significantly in the treatments of the fermentation broths of Rstps1 and Rstpp transformants, and that in the treatment of the fermentation broth of Rstre transformant visibly increased. Furthermore, the fermentation broths of the transformants of all the three genes were added to potato dextrose agar (PDA) medium for the cultivation of R. solani, as a result, the dry weight of sclerotia in each PDA plate containing the fermentation broths of Rstps1 and Rstpp transformants significantly increased compared with the control, and that of Rstre transformant obviously decreased. Finally, 178 proteins were found to interact with RSTPS1, and 16 of them were associated with ROS. Taken together, the findings suggest that all these three genes related to trehalose metabolism play important roles in the sclerotial development of R. solani AG-1 IA, and can be used as new targets for the development of novel high-efficiency fungicides for the controlling of RSB.     

Fungal pathogens andPratylenchus penetrans associated with preceding crops of clovers,winter wheat,and annual ryegrass and their influence on succeeding potato crops on Prince Edward Island

The incidence of soilborne fungal pathogens of potatoes and root lesion nematodes were determined in preceding crops of clovers, annual ryegrass, and winter wheat, in the succeeding potato crops and soils and severity of associated potato diseases recorded in two, 2-yr field trials. Differences were not evident in yearly recovery ofRhizoctonia solani, Verticil-Hum dahliae, V. alboatrum, andColletotrichum coccodes from previous crops and soils. However, overall trends were present with a higher recovery ofV. dahliae from wheat and a greater recovery ofR. solani from clover than either wheat or ryegrass. A previous crop of ryegrass resulted in lower levels ofR. solani in succeeding potato soils in both trials and after clover in one trial at the pre-harvest sampling time. In trial 1 greater numbers ofPratylenchus penetrans were recovered from clover soils than from soils of ryegrass or wheat. One instance of lower nematode recovery in potato soils following ryegrass occurred but no differences in their recovery from potato roots were observed. Potato wilt incidence was lowest after clovers. Stem rot severity was positively correlated with recovery ofR. solani andV. dahliae from potato stems and roots and negatively correlated withV. alboatrum from potato stems only. Nematode populations were not related to potato disease symptom severity ratings. Results reported herein indicate that only a portion of soilborne diseases of potatoes on Prince Edward Island may be related to previous crops produced in the 3-yr rotation selected for this study.     

The effect of crop rotations on Rhizoctonia disease of potatoes

Since 1964, replicated rotation plots in Presque Isle, Maine have continually been planted and evaluated to determine the effect of various rotation crops on tuber appearance, quality, and yield. In 1974 and 1975, the saprophytic activity ofRhizoctonia solani in the soils of each rotation combination was determined, and the tubers harvested were scored forRhizoctonia damage. Soils with a two-year rotation of oat-potato had the lowest amount of saprophytic activity byRhizoctonia and provided the lowest incidence of disease on the stems, roots, and tubers of the cultivars Kennebec and Katahdin. With the cultivar Russet Burbank, the three-year rotations generally provided the lowest incidence of disease; however, this cultivar is more susceptible to the pathogen. The plots of potatoes that were not rotated had the highest pathogen activity and the greatest disease incidence.     

Controlling parasitic nematodes and soil-borne diseases of potatoes with soil fumigation

The fumigation of soil infested with a moderate population of parasitic nematodes and fungi very effectively improved the growth, quality and yield of potatoes. The beneficial effects of fumigation with DD and Vorlex were mainly attributed to control of nematodes, particularly the meadow nematode. However, the fumigants did effectively reduce the population ofRhizoctonia solani on potato roots and “scurf” on tubers caused byR. solani. Also less vascular browning of tubers developed in fumigated soil. Thus the effects of soil fumigation were complex, affecting many microorganisms and consequently producing numerous manifestations in the growth of potatoes.     

Field study of the relative susceptibility of eleven potato (Solanum tuberosum L.) varieties and the efficacy of two fungicides against Rhizoctonia solani attack

Rhizoctonia black scurf is an economically important disease of potatoes in Tunisia and around the world. It reduces the quality and yield of potatoes and has become an important impediment for export of potatoes, especially to Europe. The information on comparative susceptibility of potato cultivars will help the growers make informed decisions regarding the management of this disease. The eleven potato cultivars used in this study showed a range of susceptibility reactions to Rhizoctonia solani but none of these was completely resistant to the disease. The Spunta variety showed the least percentage of infection of progeny tubers by R. solani sclerotia at harvest, while the varieties Nicola, Santana, Labadia, Liseta and Tango showed a high percentage of infection of progeny tubers. Seed treatment using fungicides, presently registered in Tunisia, are not effective in controlling the disease to growers’ satisfaction. Field experiments conducted to evaluate the efficacy of two newly introduced fungicides pencycuron (Monceren^®) and azoxystrobin (Amistar^®) showed that they were effective for the control of the disease. Seed potatoes treatment and in-furrow application of azoxystrobin consistently provided superior protection in two years of experimentation.     

Evaluation of potato cultivars,clones and a true seed population for resistance toRhizoctonia solani

Field and laboratory studies were conducted in 1985–1986 to evaluate the relative severity ofRhizoctonia solani damage among 26 clones and five potato (Solanum tuberosum) cultivars. A test with true potato seeds showed that resistance toR. solani could be identified in two years. One highly resistant line was identified by this method. Moderate degrees of resistance were found in lines from the USDA Potato Breeding Program (BARC) with russet types producing a higher percentage of highly resistant lines than round whites. A laboratory test revealed that apical sprout damage (nipping) was variable within and between lines and that it was an important phase of disease development to measure when determining resistance because of its effects on the resulting plant growth and production of marketable tubers. It was also determined that to accurately define resistance toR. solani in potato, all phases of the disease must be evaluated to insure against resistance “breakdown” at some point in plant development.     

Evaluation of thiabendazole and pentachloronitrobenzene for control of the Rhizoctonia disease complex on white potato (Solanum tuberosum L.)

Thiabendazole (TBZ) and pentachloronitrobenzene (PCNB) alone and in combination were laboratory, greenhouse, and field tested during a five-year period (1977-1982) for their effectiveness in reducing incidence and severity of damage caused byRhizoctonia solani to white potato (Solarium tuberosum). Growth ofR. solani on amended potato dextrose agar was prevented at 7 ppm TBZ and 80,000 ppm PCNB. Tuber-borne sclerotial germination and hyphal growth were prevented following seed treatment with 6,000 ppm TBZ, and severely inhibited by combination treatments of 12,500 or 50,000 ppm PCNB and 1,500 ppm TBZ. Seed treatments which provided the most effective field control ofR. solani were combinations of PCNB (50,000, 25,000 ppm) and TBZ (1,500 ppm), followed by TBZ alone (6,000 ppm) and PCNB alone at rates of 50,000 ppm or above.     

Effect of potato seed tuber glycoalkaloid content on subsequent infection byRhizoctonia solani

Since glycoalkaloids have been shown to have fungitoxic properties, it is important to know what impact this might have on potato plant breeding programs which select for low levels of total glycoalkaloids (TGA) in the tuber. Potato clones with TGA levels ranging between 1.6 and 32.8 mg/100 g fresh tissue were planted in two trials to evaluate the relationship between seed tuber TGA and incidence ofRhizoctonia solani infection in the developing plant. An inoculation procedure was followed in the greenhouse and field to establish uniform infection of potato seedlings. Plants were examined for lesion development on stems and stolons and assigned disease ratings. In the field, yield of malformed tubers was recorded and included in the overall disease ratings. The results reported herein indicate that TGA levels in the seed tuber are unrelated to the severity ofR. solani infection in the seedling.     

Evaluation ofLaetisaria arvalis as a biological control agent ofRhizoctonia solani on white potato

Laetisaria arvalis, a soil-inhabiting basidiomycete, reducedRhizoctonia solani infection of white potato. A greenhouse study that monitored population levels of both fungi showed that soil infested withL. arvalis suppressed the growth ofR. solani after eight wks. Populations ofL. arvalis andR. solani increased after 11 wks. when both fungi were infested simultaneously and whenL. arvalis was introduced to soil previously infested withR. solani. Five years of field studies showed that variousL. arvalis seed treatments and soil amendments reducedR. solani infection of stems and stolons and sclerotial formation on tubers equal to the standard chemical treatment of pentachloronitrobenzene (PCNB) + thiabendazole. No relationship was shown between populations ofL. arvalis andR. solani in field plot soils, but native or wild strains ofL. arvalis were shown to be present at detectable levels in field soils.     

Effect of inoculum density and quantitative PCR-based detection of Rhizoctonia solani AG-2-1 and Fusarium avenaceum on canola

Canola seedling blight, caused by Rhizoctonia solani, and Fusarium spp., can result in large yield losses to canola (Brassica napus) at high inoculum pressure. The effect of inoculum density was studied by mixing different amounts of R. solani AG-2-1 and Fusarium avenaceum into a sterilized natural soil and soil-less mix (2:1, v:v) separately, and recording seedling emergence, damping-off and seedling height within ten days after seeding; root rot severity at 12 days after seeding and seed yield at harvest on canola cultivars ‘45H29’ and ‘73-77RR’. Root rot severity increased and emergence, plant height and seed yield decreased with increased inoculum density of both R. solani and F. avenaceum. For quantification of R. solani AG-2-1, a primer and TaqMan probe set (Rs21F/Rs21R/Rs21P) was designed based on the nuclear ribosomal internal transcribed spacer (ITS) region of R. solani AG-2-1. From a conventional PCR amplification, an 88-bp product was amplified from all isolates classified as AG-2-1 with the primers Rs21F and Rs21R. No product was amplified with DNA from isolates belonging to other anastomosis groups of R. solani, other pathogens or the host plant. By using quantitative PCR, DNA amounts as low as 100 fg of R. solani AG-2-1 were detected. The quantity of DNA from soil samples with different inoculum densities estimated using qPCR was highly correlated to the number of colony-forming units (cfu) obtained from the same soil samples for both R. solani AG-2-1 and F. avenaceum.     

Survival ofHelminthosporium solani in soil andin vitro colonization of senescent plant tissue

Silver scurf, caused byHelminthosporium solani, is considered to be a tuberborne disease of potato. The ability ofH. solani to overwinter in soil was evaluated in field studies at two locations in New York during 1992. Field plots were established at sites whereH. solani infected potatoes had been produced in 1991. Plants produced from pathogen-free tubers of four cultivars yielded tubers infected with silver scurf at both locations. Up to 61% of tubers were infected in some plots. The ability ofH. solani to colonize leaf tissue was evaluated inin vitro assays. Detached leaves of ten crops were sprayed with a spore suspension ofH. solani and incubated for 20 days.H. solani colonized and sporulated on senescent leaf tissue of alfalfa, sorghum, rye, oats, corn and wheat, and only colonized senescent tissue of rapeseed, red clover and buckwheat. No growth was observed on potato leaf tissue. These results indicate that soil survival and saprophytic ability may be important in the epidemiology of silver scurf of potato.     

Maize genotype susceptibility to Rhizoctonia solani and its effect on sugar beet crop rotations

Root and crown rot is the major soil-borne fungal disease in sugar beet. In Europe, the disease is mainly caused by the anastomosis group (AG) 2-2IIIB of the basidiomycete Rhizoctonia solani (Kühn). No chemical fungicide to control the disease has been registered in Europe. Therefore, agronomic measures must be optimized to keep the disease severity below an economic damage threshold and to minimize white sugar yield losses. R. solani AG 2-2IIIB infects many other crops besides sugar beet, including maize, where it causes root rot. Sugar beet and maize are frequently grown in the same crop rotation. The proportion of cultivated maize in several European sugar beet growing areas is expected to rise due to a projected increase in demand for renewable resources over the next few years. Although the susceptibility to and tolerance of the disease varies among cultivars in both crops, little is known about the effects of cultivar susceptibility in the pre-crop on a subsequent susceptible crop. The cultivation of R. solani-resistant maize genotypes in rotation with resistant sugar beet might therefore be a useful tool in an integrated control strategy against R. solani, eliminating the need to restrict the desired crop rotation for phytosanitary reasons. A crop rotation experiment with artificially inoculated R. solani was conducted in the field to investigate the pre-crop effects of maize cultivars which differed in their susceptibility to R. solani on a susceptible sugar beet cultivar. We hypothesized that the maize genotype would influence the inoculum potential and performance of a susceptible sugar beet genotype grown after a maize pre-crop, and that this would correlate with the susceptibility of the maize genotype. The results demonstrate that the susceptibility of maize genotypes is consistent over a period of years and that cultivated maize genotypes influenced the inoculum potential measured as disease severity in sugar beet. However, disease severity in sugar beet did not correlate with the disease susceptibility of the genotype of the maize pre-crop. Possible reasons for this missing relationship might be differences in the quality of maize residues for the saprophytic survival of the pathogen or a genotype-specific alteration of the antagonistic microbial community. However, our findings showed that in the presence of maize- and sugar beet-pathogenic R. solani, the most favourable maize cultivar for a crop rotation cannot be determined solely on the basis of its resistance level against Rhizoctonia root rot.     

The effect of tuber-borne sclerotia ofRhizoctonia solani Kühn on the potato crop

Black scurf on seed tubers did not result in seed decay or stem cankers and did not affect the tuber yield in two field experiments at Ottawa and Fredericton. Seed pieces were classified into three disease categories: 0 to 1%, 1.1 to 5%, 5.1 to 15%, according to the percentage surface area covered by sclerotia ofRhizoctonia solani. The difference between disease categories, in the amount of black scurf on the seed, was perpetuated on the progeny tubers although the actual amount decreased.     

Effects of soil-borneRhizoctonia solani Kühn on yield and quality of ten potato cultivars

Summary The effects of soil-borneRhizoctonia solani on yield and quality of potato were studied by an extensive individual plant sampling procedure. From 1983 to 1986, stem canker and stolon pruning were examined in 10 768 plants growing on a sandy soil. Tuber yield and quality and haulm yield per plant were also recorded. The degree of attack byR. solani depended on the cropping frequency of potato. Severe and very severe stem and stolon attacks decreased fresh yield, dry matter yield and dry matter content of tubers and increased the number of deformed and small tubers, whereas the effect on haulm yield and stem number was small. Potato cultivars differed in susceptibility toR. solani, but yield response did not differ significantly between cultivars at the same levels of stem and stolon infection.