Cultural management of microbial community structure to enhance growth of apple in replant soils

ABSTRACT Apple replant disease typically is managed through pre-plant application of broad-spectrum soil fumigants including methyl bromide. The impending loss or restricted use of soil fumigants and the needs of an expanding organic tree fruit industry necessitate the development of alternative control measures. The microbial community resident in a wheat field soil was shown to suppress components of the microbial complex that incites apple replant disease. Pseudomonas putida was the primary fluorescent pseudomonad recovered from suppressive soil, whereas Pseudomonas fluorescens bv. III was dominant in a conducive soil; the latter developed within 3 years of orchard establishment at the same site. In greenhouse studies, cultivation of wheat in replant orchard soils prior to planting apple suppressed disease development. Disease suppression was induced in a wheat cultivar-specific manner. Wheat cultivars that enhanced apple seedling growth altered the dominant fluorescent pseudo-monad from Pseudomonas fluorescens bv. III to Pseudomonas putida. The microbial community resident in replant orchard soils after growing wheat also was suppressive to an introduced isolate of Rhizoctonia solani anastomosis group 5, which causes root rot of apple. Incorporation of high glucosinolate containing rapeseed ('Dwarf Essex') meal also enhanced growth of apple in replant soils through suppression of Rhizoc-tonia spp., Cylindrocarpon spp., and Pratylenchus penetrans. Integration of these methods will require knowledge of the impact of the biofumigant component on the wheat-induced disease-suppressive microbial community. Implementation of these control strategies for management of apple replant disease awaits confirmation from ongoing field validation trials.     

Impact of wheat cultivation on microbial communities from replant soils and apple growth in greenhouse trials

ABSTRACT Studies were conducted to assess the impact of short-term rotations of wheat on microbial community composition and growth of apple in soils from replant orchard sites. Soils from two orchards were cultivated with three successive 28-day growth cycles of 'Eltan', 'Penewawa', or 'Rely' wheat in the greenhouse and subsequently planted to 'Gala' apple seedlings. Cultivation of orchard replant soils with any of the three wheat cultivars enhanced growth of apple relative to that achieved in untreated soils. Improved growth was associated with a marked reduction in apple root infection by species of Rhizoctonia and Pythium. Populations of plant-parasitic nematodes were below damage threshold levels in these orchard soils; however, apple seedlings grown in wheat-cultivated soils had significantly lower root populations of Pratylenchus spp. than did seedlings grown in untreated soils. Growth of apple in 'Penewawa'-cultivated soils often was superior to that observed in soils planted with 'Eltan' or 'Rely'. In untreated orchard soils, fluorescent pseudomonad populations isolated from soil and the apple rhizosphere were dominated by Pseudomonas fluorescens biotype C and Pseudomonas syringae. Cultivation of replant soils with wheat induced a characteristic transformation of the fluorescent pseudomonad population, and Pseudomonas putida dominated the population of this bacterial group recovered from wheat-cultivated replant orchard soils. Results from this study suggest that use of short-term wheat cropping sequences during orchard renovation could be useful in management of replant disease and that this disease-control option may operate, in part, through modification of the fluorescent pseudomonad community.     

Identification and Pathogenicity of Rhizoctonia spp. Isolated from Apple Roots and Orchard Soils

ABSTRACT Rhizoctonia spp. were isolated from the roots of apple trees and associated soil collected in orchards located near Moxee, Quincy, East Wenatchee, and Wenatchee, WA. The anastomosis groups (AGs) of Rhizoctonia spp. isolated from apple were determined by hyphal anastomosis with tester strains on 2% water agar and, where warranted, sequence analysis of the rDNA internal transcribed spacer region and restriction analysis of an amplified fragment from the 28S ribosomal RNA gene were used to corroborate these identifications. The dominant AG of R. solani isolated from the Moxee and East Wenatchee orchards were AG 5 and AG 6, respectively. Binucleate Rhizoctonia spp. were recovered from apple roots at three of four orchards surveyed and included isolates of AG-A, -G, -I, -J, and -Q. In artificial inoculations, isolates of R. solani AG 5 and AG 6 caused extensive root rot and death of 2- to 20-week-old apple transplants, providing evidence that isolates of R. solani AG 6 can be highly virulent and do not merely exist as saprophytes. The effect of binucleate Rhizoctonia spp. on growth of apple seedlings was isolate-dependent and ranged from growth enhancement to severe root rot. R. solani AG 5 and AG 6 were isolated from stunted trees, but not healthy trees, in an orchard near Moxee, WA, that exhibited severe symptoms of apple replant disease, suggesting that R. solani may have a role in this disease complex.     

Transformation of soil microbial community structure and rhizoctonia-suppressive potential in response to apple roots

ABSTRACT Changes in the composition of soil microbial communities and relative disease-suppressive ability of resident microflora in response to apple cultivation were assessed in orchard soils from a site possessing trees established for 1 to 5 years. The fungal community from roots of apple seedlings grown in noncultivated orchard soil was dominated by isolates from genera commonly considered saprophytic. Plant-pathogenic fungi in the genera Phytophthora, Pythium, and Rhizoctonia constituted an increasing proportion of the fungal community isolated from seedling roots with increasing orchard block age. Bacillus megaterium and Burkholderia cepacia dominated the bacterial communities recovered from noncultivated soil and the rhizosphere of apple seedlings grown in orchard soil, respectively. Populations of the two bacteria in their respective habitats declined dramatically with increasing orchard block age. Lesion nematode populations did not differ among soil and root samples from orchard blocks of different ages. Similar changes in microbial communities were observed in response to planting noncultivated orchard soil to five successive cycles of 'Gala' apple seedlings. Pasteurization of soil had no effect on apple growth in noncultivated soil but significantly enhanced apple growth in third-year orchard block soil. Seedlings grown in pasteurized soil from the third-year orchard block were equal in size to those grown in noncultivated soil, demonstrating that suppression of plant growth resulted from changes in the composition of the soil microbial community. Rhizoctonia solani anastomosis group 5 (AG 5) had no effect on growth of apple trees in noncultivated soil but significantly reduced the growth of apple trees in soil from third-year orchard soil. Changes in the ability of the resident soil microflora to suppress R. solani AG 5 were associated with reductions in the relative populations of Burkholderia cepacia and Pseudomonas putida in the rhizosphere of apple.     

Mechanism of action and efficacy of seed meal-induced pathogen suppression differ in a brassicaceae species and time-dependent manner

ABSTRACT The effect of seed meals derived from Brassica juncea, B. napus, or Sinapis alba on suppression of soilborne pathogens inciting replant disease of apple was evaluated in greenhouse trials. Regardless of plant source, seed meal amendment significantly improved apple growth in all orchard soils; however, relative differences in pathogen suppression were observed. All seed meals suppressed root infection by native Rhizoctonia spp. and an introduced isolate of Rhizoctonia solani AG-5, though B. juncea seed meal often generated a lower level of disease control relative to other seed meal types. When introduction of the pathogen was delayed until 4 to 8 weeks post seed meal amendment, disease suppression was associated with proliferation of resident Streptomyces spp. and not qualitative or quantitative attributes of seed meal glucosinolate content. Using the same experimental system, when soils were pasteurized prior to pathogen infestation, control of R. solani was eliminated regardless of seed meal type. In the case of B. juncea seed meal amendment, the mechanism of R. solani suppression varied in a temporal manner, which initially was associated with the generation of allylisothiocyanate and was not affected by soil pasteurization. Among those tested, only B. juncea seed meal did not stimulate orchard soil populations of Pythium spp. and infection of apple roots by these oomycetes. Although application of B. napus seed meal alone consistently induced an increase in Pythium spp. populations, no significant increase in Pythium spp. populations was observed in response to a composite B. juncea and B. napus seed meal amendment. Suppression of soil populations and root infestation by Pratylenchus spp. was dependent upon seed meal type, with only B. juncea providing sustained nematode control. Collectively, these studies suggest that use of a composite B. juncea and B. napus seed meal mixture can provide superior control of the pathogen complex inciting apple replant disease relative to either seed meal used alone.     

Identification and quantification of Rhizoctonia solani and R. oryzae using real-time polymerase chain reaction

Rhizoctonia solani and R. oryzae are the principal causal agents of Rhizoctonia root rot in dryland cereal production systems of the Pacific Northwest. To facilitate the identification and quantification of these pathogens in agricultural samples, we developed SYBR Green I-based real-time quantitative-polymerase chain reaction (Q-PCR) assays specific to internal transcribed spacers ITS1 and ITS2 of the nuclear ribosomal DNA of R. solani and R. oryzae. The assays were diagnostic for R. solani AG-2-1, AG-8, and AG-10, three genotypes of R. oryzae, and an AG-I-like binucleate Rhizoctonia species. Quantification was reproducible at or below a cycle threshold (Ct) of 33, or 2 to 10 fg of mycelial DNA from cultured fungi, 200 to 500 fg of pathogen DNA from root extracts, and 20 to 50 fg of pathogen DNA from soil extracts. However, pathogen DNA could be specifically detected in all types of extracts at about 100-fold below the quantification levels. Soils from Ritzville, WA, showing acute Rhizoctonia bare patch harbored 9.4 to 780 pg of R. solani AG-8 DNA per gram of soil.. Blastn, primer-template duplex stability, and phylogenetic analyses predicted that the Q-PCR assays will be diagnostic for isolates from Australia, Israel, Japan, and other countries.     

Effect of inoculum density and soil tillage on the development and severity of rhizoctonia root rot

Rhizoctonia spp. cause substantial yield losses in direct-seeded cereal crops compared with conventional tillage. To investigate the mechanisms behind this increased disease, soils from tilled or direct-seeded fields were inoculated with Rhizoctonia spp. at population densities from 0.8 to 250 propagules per gram and planted with barley (Hordeum vulgare). The incidence and severity of disease did not differ between soils with different tillage histories. Both R. solani AG-8 and R. oryzae stunted plants at high inoculum densities, with the latter causing pre-emergence damping-off. High inoculum densities of both species stimulated early production of crown roots in barley seedlings. Intact soil cores from these same tilled and direct-seeded fields were used to evaluate the growth of Rhizoctonia spp. from colonized oat seeds. Growth of R. oryzae was not affected by previous tillage history. However, R. solani AG-8 grew more rapidly through soil from a long-term direct-seeded field compared to tilled soils. The differential response between these two experiments (mixed, homogenized soil versus intact soil) suggests that soil structure plays a major role in the proliferation of R. solani AG-8 through soils with different tillage histories.     

Incidence of soil-borne plant pathogens isolated from barley and winter wheat, and other crops in the rotation, on Prince Edward Island

Barley and winter wheat were sampled over 3 years to characterize soil-borne organisms involved with cereal crown and root disease complexes. Winter wheat crowns were infected more often by Fusarium avenaceum and F. sambucinum than barley crowns. Bipolaris sorokiniana and F. graminearum were more common in crown tissue of barley underseeded with clover than in winter wheat crowns. In roots, the incidence of F. avenaceum was highest in winter wheat. In soil, populations of Rhizoctonia solani and F. sambucinum were higher in winter wheat than barley underseeded with ryegrass. Stunt nematodes (Tylenchorhynchus spp.) were greatest in winter wheat and barley underseeded with ryegrass. The incidence of F. avenaceum in roots of winter wheat correlated positively with the severity of crown and root rot symptoms. The severity of barley crown and root rot correlated positively with the incidence of R. solani in crowns, F. avenaceum in crowns and roots, B. sorokiniana in roots, and numbers of stunt nematodes in the soil. The incidence of soil-borne organisms was also recorded in annual ryegrass. soybean, potato, pea, and clover which were grown in rotation with barley and winter wheat. The incidence of R. solani and F. avenaceum was highest in clover crown tissue, and F. sambucinum was prevalent in soybean hypocotyls. Soil population levels of R. solani, F. avenaceum , and stunt nematodes were greatest in ryegrass and pea soil just prior to ploughing down or harvesting these crops, respectively.     

Molecular tools to investigate Rhizoctonia solani distribution in soil

Real-time PCR protocols were developed to detect and discriminate 11 anastomosis groups (AGs) of Rhizoctonia solani using ribosomal internal transcribed spacer (ITS) regions (AG-1-IA, AG-1-IC, AG-2-1, AG-2-2, AG-4HGI+II, AG-4HGIII, AG-8) or β-tubulin (AG-3, AG-4HGII, AG-5 and AG-9) sequences. All real-time assays were target group specific, except AG-2-2, which showed a weak cross-reaction with AG-2^tabac. In addition, methods were developed for the high throughput extraction of DNA from soil and compost samples. The DNA extraction method was used with the AG-2-1 assay and shown to be quantitative with a detection threshold of 10⁻⁷ g of R. solani per g of soil. A similar DNA extraction efficiency was observed for samples from three contrasting soil types. The developed methods were then used to investigate the spatial distribution of R. solani AG-2-1 in field soils. Soil from shallow depths of a field planted with Brassica oleracea tested positive for R. solani AG-2-1 more frequently than soil collected from greater depths. Quantification of R. solani inoculum in field samples proved challenging due to low levels of inoculum in naturally occurring soils. The potential uses of real-time PCR and DNA extraction protocols to investigate the epidemiology of R. solani are discussed.     

Rhizoctonia species and anastomosis groups isolated from barley and wheat in Erzurum, Turkey

Ninety-eight isolates of Rhizoctonia spp. were obtained from barley and wheat grown in Erzurum, Turkey. Of these, 78% were Rhizoctonia solani (AG-2 type 1, AG-3, AG-4, AG-5 and AG-11), 10% were binucleate Rhizoctonia (AG-I and AG-K) and the remainder were Waitea circinata var circinata ( Rhizoctonia sp.). Among the binucleate Rhizoctonia , AG-I was not recovered from barley. In pathogenicity tests on barley and wheat, the highest disease severity was caused by isolates of AG-4 and AG-11, whereas isolates of AG-2 type 1, AG-3, AG-5 and W. c . var  circinata were moderately virulent. Isolates of binucleate Rhizoctonia were all nonpathogenic. This is the first report of R. solani AG-11 and W. c . var  circinata from Turkey.     

Bacillus sp. L324-92 for Biological Control of Three Root Diseases of Wheat Grown with Reduced Tillage

ABSTRACT Strain L324-92 is a novel Bacillus sp. with biological activity against three root diseases of wheat, namely take-all caused by Gaeumannomyces graminis var. tritici, Rhizoctonia root rot caused by Rhizoctonia solani AG8, and Pythium root rot caused mainly by Pythium irregulare and P. ultimum, that exhibits broad-spectrum inhibitory activity and grows at temperatures from 4 to 40 degrees C. These three root diseases are major yieldlimiting factors for wheat in the U.S. Inland Pacific Northwest, especially wheat direct-drilled into the residue of a previous cereal crop. Strain L324-92 was selected from among approximately 2,000 rhizosphere/rhizoplane isolates of Bacillus species isolated from roots of wheat collected from two eastern Washington wheat fields that had long histories of wheat. Roots were washed, heat-treated (80 degrees C for 30 min), macerated, and dilution-plated on (1)/(10)-strength tryptic soy agar. Strain L324-92 inhibited all isolates of G. graminis var. tritici, Rhizoctonia species and anastomosis groups, and Pythium species tested on agar at 15 degrees C; provided significant suppression of all three root diseases at 15 degrees C in growth chamber assays; controlled either Rhizoctonia root rot, takeall, or both; and increased yields in field tests in which one or more of the three root diseases of wheats were yield-limiting factors. The ability of L324-92 to grow at 4 degrees C probably contributes to its biocontrol activity on direct-drilled winter and spring wheat because, under Inland Northwest conditions, leaving harvest residues of the previous crop on the soil surface keeps soils cooler compared with tilled soils. These results suggest that Bacillus species with desired traits for biological control of wheat root diseases are present within the community of wheat rhizosphere microorganisms and can be recovered by protocols developed earlier for isolation of fluorescent Pseudomonas species effective against take-all.     

Anastomosis group and pathogenicity of isolates of Rhizoctonia solani from potato crops in South Australia

Isolates of Rhizoctonia collected from the stems, roots, tuber sclerotia and soil of potato crops in Virginia and Lenswood, South Australia, were identified to anastomosis groups (AG). Of the 301 multinucleate isolates of Rhizoctonia solani tested, 90% were AG-3, 7% were AG-4 and 2% were AG-5; 12 isolates were binucleate Rhizoctonia spp. This is the first report of isolates of AG-4 and AG-5 causing disease in potato crops in South Australia. All AG-3, AG-4 and AG-5 isolates tested caused rhizoctonia disease symptoms on the potato cultivar Coliban in pathogenicity trials conducted under glasshotise conditions. Both AG-3 and AG-5 isolates caused black scurf and stem cankers, although symptoms of black scurf were less severe with AG-5. AG-4 isolates produced the most severe stem and stolon cankers of all isolates tested. The pathogenicity of tuber-borne inoculum was confirmed by growing plants from sclerotia-infested tubers. AG-8 isolates from diseased barley and wheat produced severe root cankers and caused loss of feeder roots on inoculated potato plants. Results suggest that rhizoctonia disease in potato fields in South Australia is caused by a combination of different anastomosis groups and this has important implications for crop rotations.     

Elucidation of the microbial complex having a causal role in the development of apple replant disease in washington

ABSTRACT Systematic studies were conducted to elucidate the role of different soil microbial groups in the development of apple replant disease. Populations of targeted microorganisms were reduced by the application of semiselective biocides and soil pasteurization. Bacteria were not implicated in the disease, because application of the antibiotic chloramphenicol reduced soil populations of bacteria but failed to improve growth of apple transplants, while enhanced growth was achieved at pasteurization temperatures that did not alter attributes of the bacterial community recovered from apple roots. Populations of Pratylenchus penetrans were below the damage threshold level in eight of nine orchards surveyed, and nematicide applications failed to enhance apple growth in four of five replant soils tested, indicating that plant parasitic nematodes have a minor role or no role in disease development. Application of the fungicide difenconazole or metalaxyl enhanced growth of apple in all five replant soils, as did fludioxinil in the two soils tested. Soil pasteurization enhanced growth of apple and resulted in specific changes in the composition of the fungal community isolated from the roots of apple seedlings grown in these treated soils. Cylindrocarpon destructans, Phytophthora cactorum, Pythium spp., and Rhizoctonia solani were consistently isolated from symptomatic trees in the field and were pathogenic to apple. However, the composition of the Pythium and Rhizoctonia component and the relative contribution of any one component of this fungal complex to disease development varied among the study orchards. These findings clearly demonstrate that fungi are the dominant causal agents of apple replant disease in Washington state.     

Detecting Migrants in Populations of Rhizoctonia solani Anastomosis Group 3 from Potato in North Carolina Using Multilocus Genotype Probabilities

ABSTRACT The relative contribution of migration of Rhizoctonia solani anastomosis group 3 (AG-3) on infested potato seed tubers originating from production areas in Canada, Maine, and Wisconsin (source population) to the genetic diversity and structure of populations of R. solani AG-3 in North Carolina (NC) soil (recipient population) was examined. The frequency of alleles detected by multilocus polymerase chain reaction-restriction fragment length polymorphisms, heterozygosity at individual loci, and gametic phase disequilibrium between all pairs of loci were determined for subpopulations of R. solani AG-3 from eight sources of potato seed tubers and from five soils in NC. Analysis of molecular variation revealed little variation between seed source and NC recipient soil populations or between subpopulations within each region. Analysis of population data with a Bayesian-based statistical method previously developed for detecting migration in human populations suggested that six multilocus genotypes from the NC soil population had a statistically significant probability of being migrants from the northern source population. The one-way (unidirectional) migration of genotypes of R. solani AG-3 into NC on infested potato seed tubers from Canada, Maine, and Wisconsin provides a plausible explanation for the lack of genetic subdivision (differentiation) between populations of the pathogen in NC soils or between the northern source and the NC recipient soil populations.     

A comparison of controlled environment and field trials for detection of resistance in cereal cultivars to root rot caused by Rhizoctonia solani

A screening test was developed to study the effect of different quantities of inoculum of Rhizoctonia solani on root rot and growth of cultivars of wheat, barley, oats, rye and triticale in a controlled environment and the field in two seasons. The ranking of relative susceptibility to disease of cultivars within a cereal differed between the controlled environment and field experiments, and between seasons in the field. The ranking of relative susceptibility between cereals also differed between experiments. In the controlled-environment experiment, wheat had the highest disease rating, followed by oats, barley, triticale and rye, but in the field barley had the highest rating followed by wheat and then oats. The overall differences between the cereals were small.     

Induction of Soil Suppressiveness Against Rhizoctonia solani by Incorporation of Dried Plant Residues into Soil

ABSTRACT Suppressive effects of soil amendment with residues of 12 cultivars of Brassica rapa on damping-off of sugar beet were evaluated in soils infested with Rhizoctonia solani. Residues of clover and peanut were tested as noncruciferous controls. The incidence of damping-off was significantly and consistently suppressed in the soils amended with residues of clover, peanut, and B. rapa subsp. rapifera 'Saori', but only the volatile substance produced from water-imbibed residue of cv. Saori exhibited a distinct inhibitory effect on mycelial growth of R. solani. Nonetheless, disease suppression in such residue-amended soils was diminished or nullified when antibacterial antibiotics were applied to the soils, suggesting that proliferation of antagonistic bacteria resident to the soils were responsible for disease suppression. When the seed (pericarps) colonized by R. solani in the infested soil without residues were replanted into the soils amended with such residues, damping-off was suppressed in all cases. In contrast, when seed that had been colonized by microorganisms in the soils containing the residues were replanted into the infested soil, damping-off was not suppressed. The evidence indicates that the laimosphere, but not the spermosphere, is the site for the antagonistic microbial interaction, which is the chief principle of soil suppressiveness against Rhizoctonia damping-off.     

Aetiology of Rhizoctonia in sheath blight of maize in Sichuan

Rhizoctonia isolates obtained from maize grown in commercial fields in 33 representative counties (or cities) in Sichuan province in China were characterized according to colony morphology, hyphal anastomosis and pathogenicity. Of 141 isolates, 116 were identified as R. solani , 23 as R. zeae and two as binucleate Rhizoctonia . The isolates of R. solani were assigned to four anastomosis groups (AG): AG-1-IA (101 isolates, accounting for 71.6% of the total), AG-1-IB (2, 1.4%), AG-4 (9, 6.4%) and AG-5 (4, 2.8%). The two isolates of binucleate Rhizoctonia belonged to AG-K. On maize, isolates of AG-1-IA caused typical sheath blight symptoms. Lesions produced by isolates of AG-4, AG-5, AG-1-IB and AG-K were darker than those of AG-1-IA. Rhizoctonia zeae usually caused discontinuous lesions with a dark brown margin and a brown centre on the leaf sheaths, as well as ear rot. Isolates of AG-1-IA were the most virulent to maize, with an average lesion length of approximately 15 cm. Isolates of R. zeae produced lesions approximately 12 cm long, while those of AG-4, AG-5, AG-1-IB and AG-K were progressively shorter. On potato dextrose agar (PDA; pH 6.4), the minimum temperature for mycelial growth of R. zeae isolates was 14–18°C, the maximum 38–40°C and optimum 30°C. Isolates of R. zeae did not grow on PDA (28°C) at pH 2.0, the optimum for growth being pH 6.4.     

新疆北疆棉田立枯丝核菌不同菌丝融合群致病力的研究

从新疆北疆棉区采集了典型的棉花立枯病病苗及棉田土标样686份,按常规分离方法分离得到399个分离物,从中鉴定出272个纯化的立枯丝核菌(Rhizoctonia solani Kühn)菌株。用标准菌株,通过载玻片菌丝融合试验测定,将纯化的272个菌株划归为3个菌丝融合群:即AG-2、AG-4和AG-5,分别占总菌株的6.24%、84.2%和1.1%。另有23个菌株不与任何标准菌株融合,占8.46%,说明新疆北疆棉田立枯丝核菌的优势菌系是多核丝核菌的AG-4融合群。通过从10种不同配方培养基中筛选效果好的麦芽蛋白胨(MPDA)配方培养基(Ⅱ)进行对峙培养,将纯化获得的272个丝核菌菌株,划分为6个不同的营养亲和群。将多核的不同菌丝融合群及其各营养亲和群代表菌株在3种不同主栽棉花品种上(每品种都加不接菌的对照)进行温室盆栽致病力测定,结果以AG-4对棉花的致病力最强,其次是非融合类,AG-2和AG-5虽致病,但并不致死苗,其平均病指数分别为94.9、81.4、53.1、43.5。     

Influence of glyphosate on Rhizoctonia and Fusarium root rot in sugar beet

This study tests the effect of glyphosate application on disease severity in glyphosate-resistant sugar beet, and examines whether the increase in disease is fungal or plant mediated. In greenhouse studies of glyphosate-resistant sugar beet, increased disease severity was observed following glyphosate application and inoculation with certain isolates of Rhizoctonia solani Kuhn and Fusarium oxysporum Schlecht. f. sp. betae Snyd. & Hans. Significant increases in disease severity were noted for R. solani AG-2-2 isolate R-9 and moderately virulent F. oxysporum isolate FOB13 on both cultivars tested, regardless of the duration between glyphosate application and pathogen challenge, but not with highly virulent F. oxysporum isolate F-19 or an isolate of R. solani AG-4. The increase in disease does not appear to be fungal mediated, since in vitro studies showed no positive impact of glyphosate on fungal growth or overwintering structure production or germination for either pathogen. Studies of glyphosate impact on sugar beet physiology showed that shikimic acid accumulation is tissue specific and the rate of accumulation is greatly reduced in resistant cultivars when compared with a susceptible cultivar. The results indicate that precautions need to be taken when certain soil-borne diseases are present if weed management for sugar beet is to include post-emergence glyphosate treatments.     

新疆11种豆科作物立枯丝核菌菌丝融合群及营养亲和群研究

从新疆11种豆科作物病株上或病株根围土样中分离纯化出250个立枯丝核菌（Rhizoctonia DC）,番红O KOH染色后观察细胞核数目,经测试全部菌株均为多核,用标准菌株测定融合群, 250个菌株分属为AG 1、AG 2、AG 3、AG 4和AG 5共5个融合群,出现频率分别为16.4%、33.2%、0.4%、32.4%和17.6%,营养亲合群判别结果表明,AG 1、AG 2、AG 4和AG 5下各有2个VCG,说明新疆豆科作物立枯丝核菌各主要菌丝融合群内均有不同程度的分化。