Bare Patch Disease Research Articles

FISHing in fungi: Visualisation of mushroom virus X in the mycelium of Agaricus bisporus by fluorescence in situ hybridisation

Agaricus bisporus is a commercial mushroom crop susceptible to a disease caused by a complex of viruses known collectively as mushroom virus X (MVX). Symptoms of MVX include bare patches and mushroom cap discolouration (browning) in the fruiting bodies, phenotypes associated with the viruses AbV6 and AbV16, respectively. Limited understanding exists of the localisation and mobilisation of these viruses within the mycelium of A. bisporus. To this end, a non-destructive fluorescence in situ hybridisation (FISH) method was developed for in situ targeting of AbV6 and AbV16 in A. bisporus mycelium. An MVX strain associated with the bare patch disease phenotype revealed predominantly high signal towards the growing edges of cultures when probed for AbV6, with a ‘halo-effect’ of high signal intensity around putative vacuoles. An MVX strain associated with the browning disease phenotype showed high signal intensities within reticulating networks of hyphae in a highly compartmentalised manner when probed for AbV16. Localisation of the two viruses in MVX-infected cultures appears independent, as both viruses were found in completely discrete areas of the mycelium in differential patterns. FISH detected low level presence of the two viruses, AbV6 and AbV16 in a number of cultures which had tested negative for the viruses by RT-PCR. This suggests that FISH may be more sensitive at detecting viruses at low levels than molecular methods. This study demonstrates that FISH is a powerful tool in the field of mycovirology.

Role of bacterial communities in the natural suppression of Rhizoctonia solani bare patch disease of wheat (Triticum aestivum L.).

Rhizoctonia bare patch and root rot disease of wheat, caused by Rhizoctonia solani AG-8, develops as distinct patches of stunted plants and limits the yield of direct-seeded (no-till) wheat in the Pacific Northwest of the United States. At the site of a long-term cropping systems study near Ritzville, WA, a decline in Rhizoctonia patch disease was observed over an 11-year period. Bacterial communities from bulk and rhizosphere soil of plants from inside the patches, outside the patches, and recovered patches were analyzed by using pyrosequencing with primers designed for 16S rRNA. Taxa in the class Acidobacteria and the genus Gemmatimonas were found at higher frequencies in the rhizosphere of healthy plants outside the patches than in that of diseased plants from inside the patches. Dyella and Acidobacteria subgroup Gp7 were found at higher frequencies in recovered patches. Chitinophaga, Pedobacter, Oxalobacteriaceae (Duganella and Massilia), and Chyseobacterium were found at higher frequencies in the rhizosphere of diseased plants from inside the patches. For selected taxa, trends were validated by quantitative PCR (qPCR), and observed shifts of frequencies in the rhizosphere over time were duplicated in cycling experiments in the greenhouse that involved successive plantings of wheat in Rhizoctonia-inoculated soil. Chryseobacterium soldanellicola was isolated from the rhizosphere inside the patches and exhibited significant antagonism against R. solani AG-8 in vitro and in greenhouse tests. In conclusion, we identified novel bacterial taxa that respond to conditions affecting bare patch disease symptoms and that may be involved in suppression of Rhizoctonia root rot and bare batch disease.

Eight years of annual no-till cropping in Washington's winter wheat-summer fallow region

The tillage-based winter wheat ( Triticum aestivum L.)-summer fallow (WW-SF) cropping system has dominated dryland farming in the Pacific Northwest USA for 125 years. We conducted a large-scale multidisciplinary 8-year study of annual (i.e., no summer fallow) no-till cropping systems as an alternative to WW-SF. Soft white and hard white classes of winter and spring wheat, spring barley ( Hordeum vulgare L.), yellow mustard ( Brassica hirta Moench), and safflower ( Carthamus tinctorius L.) were grown in various rotation combinations. Annual precipitation was less than the long-term average of 301 mm in 7 out of 8 years. Rhizoctonia bare patch disease caused by the fungus Rhizoctonia solani AG-8 appeared in year 3 and continued through year 8 in all no-till plots. All crops were susceptible to rhizoctonia, but bare patch area in wheat was reduced, and grain yield increased, when wheat was grown in rotation with barley every other year. Remnant downy brome ( Bromus tectorum L.) weed seeds remained dormant for 6 years and longer to heavily infest recrop winter wheat. There were few quantifiable changes in soil quality due to crop rotation, but soil organic carbon (SOC) increased in the surface 0–5 cm depth with no-till during the 8 years to approach that found in undisturbed native soil. Annual no-till crop rotations experienced lower average profitability and greater income variability compared to WW-SF. Yellow mustard and safflower were not economically viable. Continuous annual cropping using no-till provides excellent protection against wind erosion and shows potential to increase soil quality, but the practice involves high economic risk compared to WW-SF. This paper provides the first comprehensive multidisciplinary report of long-term alternative annual no-till cropping systems research in the low-precipitation region of the Pacific Northwest.

Reduction of Rhizoctonia Bare Patch in Wheat with Barley Rotations.

Rhizoctonia bare patch caused by Rhizoctonia solani AG-8 is a major fungal root disease in no-till cropping systems. In an 8-year experiment comparing various dryland no-till cropping systems near Ritzville, WA, Rhizoctonia bare patch first appeared in year 3 and continued unabated through year 8. Crop rotation had no effect on bare patch during the first 5 years. However, from years 6 to 8, both soft white and hard white classes of spring wheat (Triticum aestivum L.) grown in a 2-year rotation with spring barley (Hordeum vulgare L.) had an average of only 7% of total land area with bare patches compared with 15% in continuous annual soft white wheat or hard white wheat (i.e., monoculture wheat). In years 6 to 8, average grain yield of both soft white wheat and hard white wheat were greater (P < 0.001) when grown in rotation with barley than in monoculture. Although both classes of wheat had less bare patch area and greater grain yield when grown in rotation with barley, monoculture hard white wheat was more severely affected by Rhizoctonia than soft white wheat. Soil water levels were higher in bare patches, indicating that roots of healthy cereals did not grow into or underneath bare patch areas. This is the first documentation of suppression of Rhizoctonia bare patch disease in low-disturbance no-till systems with rotation of cereal crops.

First Report of Rhizoctonia solani AG-4 and AG-2-2 on Lupinus angustifolius in Canada.

Narrow-leaved lupine (Lupinus angustifolius L.) is grown as a grain legume crop in Australia and Europe where it is used as feedstock in the livestock and aquaculture industries. During July 2003, a stem rot disease was observed in narrow-leaved lupine (cv. Arabella) plants in a research plot at the Crop Diversification Centre North (CDCN), Edmonton, Alberta, Canada. The disease was also found on cv. Rose at the CDCN and cv. Arabella in experimental fields near Devon, Ellerslie, and Westlock, Alberta during the late spring of 2004. Diseased plants showed dark brown-to-black stems with sunken and constricted lesions at the soil level. Young leaves became shrunken and twisted and seedlings collapsed. Rhizoctonia solani was consistently isolated from lesions on taproots and basal stems of diseased plants. Colonies of cream-colored mycelia grew close to the surface of potato dextrose agar (PDA). Most sclerotia formed inside the medium. Agar disks (1 cm in diameter) of isolates LP-11Bb, LP-24C, and LP-25C were attached to the opposite sides of basal stems (180° apart) of 1-month-old lupine seedlings (cv. Arabella). Inoculated plants were incubated for 2 days in black plastic bags under a greenhouse bench at approximately 20°C. All isolates caused brown lesions on the lower stems (extending up to 7 cm above ground level), girdling, and root rot. Plants wilted within 7 to 10 days after inoculation, and aerial mycelia appeared on the basal stems. R. solani was reisolated from the infected crown tissues using PDA to complete Koch's postulates. The isolates were paired with AG tester strains of R. solani. Isolates LP-11Bb and LP-24C were identified as AG-4 while isolate LP-25C was identified as AG-2-2. In another trial, eight isolates of R. solani (unknown AG types) were tested for virulence on L. angustifolius cv. Arabella using the inoculation method described above. All isolates were pathogenic, and disease severity that was based on a 0 to 4 scale ranged from 2.7 to 3.2. The most virulent strain was LP-24C, which caused a 77% loss in fresh weight compared with the noninoculated control plants. R. solani AG-8 is associated with Rhizoctonia disease of lupine in Australia (1) and also causes bare patch disease of wheat. To our knowledge, this is the first report of R. solani on lupine in Canada. This disease could have a significant impact on the commercial production of lupine in Alberta. Reference: (1) M. W. Sweetingham et al. Pages 466-486 in: Advances in Lupin Disease Management in Australia. Proc. Int. Lupin Conf., 8th. G. D. Hill, ed. International Lupin Association, Canterbury, New Zealand, 1999.

Influence of depth of soil disturbance on root growth dynamics of wheat seedlings associated with Rhizoctonia solani AG-8 disease severity in sandy and loamy sand soils of Western Australia

Rhizoctonia root rot (bare-patch disease) of cereals caused by Rhizoctonia solani AG-8 is reported to become more severe when the crop is sown under no-tillage practice. Disturbing the soil with tillage is an effective way to reduce the impact of disease. Glasshouse and field studies were conducted to evaluate the effects of the depth and intensity of soil disturbance and soil moisture conditions at the time of disturbance on rhizoctonia root rot of wheat ( Triticum aestivum L. cv. Aroona) seedlings. Results of glasshouse studies showed that intensity of soil disturbance was important for reducing the disease if the soil was disturbed at the surface (0–5 cm). Disturbing the soil at 10–20 cm below the surface without disturbing the surface soil proved to be more effective in reducing the disease than surface disturbance even when the soil was disturbed at lower intensity. Total wheat root length and rooting depth increased when soil below the seed was disturbed. Disturbance of wet soil before sowing resulted in less reduction of root and shoot weights following inoculation with R. solani AG-8 than in relatively dry soil. Similar results were obtained from a field study conducted near Newdegate (WA, Australia) on a Orthic Solonetz soil (sandy loam), with tillage of the field soil 10–15 cm below the surface producing maximum dry root and shoot weights in a rhizoctonia patch area mapped in the previous year.

Suppression of Rhizoctonia solani anastomosis group 8 in Australia and its biological nature

The nature of suppression in a field soil showing a decline in the Rhizoctonia barepatch disease of wheat (causal agent Rhizoctonia solani AG-8), in a minimum tillage system in southern Australia was investigated. The suppressive characteristics of the soil could be transferred to an autoclaved or pasteurised soil by adding 10% (w/w) of the unsterilised soil. This resulted in less disease following inoculation with R. solani AG-8. No transfer of suppression was observed when non-suppressive soil from an adjacent trial was added to the autoclaved or pasteurised soil. Gamma irradiation or pasteurisation at 60, 70 or 80°C for 30 min eliminated both the ability of the soil to suppress disease and also differences in the soil microflora of suppressive soil and non-suppressive soil observed in untreated or 50°C steam pasteurised soils. This is the first report of biologically-based suppression of this root rotting disease of wheat caused by R. solani AG-8.

Rhizoctonia Bare Patch of Cereals: An Australian Perspective.

Rhizoctonia bare patch disease, caused by Rhizoctonia solani Kuhn AG-8, was first described in Australia in the late 1920s (11,53). Since then, it has been reported in England (7). Canada (1), Scotland (32), and the United States (61). R. solani AG-8 has been confirmed as the causal organism in all locations except Canada…

DNA probe specific to Rhizoctonia solani anastomosis group 8

A DNA probe was developed which hybridizes strongly with DNAs of all isolates of Rhizoctonia solani anastomosis group 8 (AG-8) tested. The probe hybridized only weakly or not at all with DNA of isolates from other anastomosis groups. The results presented here indicate that the probe recognizes a dispersed repeated DNA element in AG-8 isolates and that genetic variation occurs within AG-8. Preliminary experiments show that the repeated unit is at least 1·8 kbp in length and that sequence variation occurs between different repeat units. The probe has the potential to further our understanding of the population dynamics of bare patch disease in cereal crops.

Anastomosis group (AG) affinity of pectic isozyme (zymogram) groups (ZG) of Rhizoctonia solani from the Western Australian cereal-belt

Over 4250 isolates of R. solani obtained from the cereal growing belt of Western Australia have been characterized using pectic isozyme patterns (zymograms). With few exceptions, all can be placed in one or other of 10 zymogram groups (ZG), four of which (ZG1-1, ZG1-2, ZG1-4, and ZG1-5) have previously been identified as members of anastomosis group AG-8 and cause rhizoctonia bare patch disease of cereals, legumes and forage pastures. Another (ZG5) has been identified as AG-2-1. The latter identification is confirmed in this study and the anastomosis group affinity of the remaining five ZGs reported. ZG4 and ZG10 are placed in AG-2-2, ZG6 in AG-2-1 and ZG9 in AG-10. ZG3, although it reacts in a very limited way with some isolates of AG-2-1 and AG-8, appears to represent an as yet unreported AG. The potential value of zymograms as an aid to disease management and as a research tool for identifying sub-groups within AGs is discussed. Integration of the AG and ZG systems, and use of ZGs to subdivide AGs is also discussed.

Field evidence for reduced severity of Rhizoctonia bare-patch disease of wheat, due to the presence of the earthworms Aporrectodea Rosea and Aporrectodea Trapezoides

A study demonstrated the ability of the earthworms Aporrectodea rosea and Aporrectodea trapezoides (added at an equivalent density of 100 or 300 m −2) to reduce the disease severity of Rhizoctonia soiani Kühn on wheat in the field. In a calcareous sandy loam artificially infested with R. solani, the addition of these earthworms caused a significant ( P = 0.02) reduction of the Rhizoctonia root disease rating and had a significant ( P = 0.01) positive effect on shoot weight. Neither earthworm number or earthworm species had a significant ( P < 0.05) effect on root disease rating or shoot weight. In a red-brown earth soil artificially infested with R. solani, neither the presence of these earthworms, earthworm species nor earthworm number influenced root disease rating. However, under these conditions the addition of earthworms ( P = 0.01) had a significant positive effect upon shoot weight. To our knowledge these results demonstrate for the first time, the potential of earthworms to contribute to the disease suppression of a cropping soil in a field situation.

Association of Rhizoctonia strains with bare patch disease of wheat in Western Australia

Rhizoctonia‐like fungi were isolated from the roots of diseased wheat plants sampled from the centre and periphery of three bare patches, and from apparently healthy plants from outside the patches. Of the isolates recovered, 81% were multinucleate and belonged to R. solani anastomosis group 8, and pectic zymogram group 1‐1; the remaining isolates were binucleate Rhizoctonia spp. The multinucleate isolates could be grouped into highly virulent, intermediately virulent, and weakly virulent types. The binucleate isolates were all non‐pathogenic. The multinucleate isolates were obtained at a significantly higher frequency from plants within the patches compared with outside the patches, and with the exception of a single isolate, the highly virulent isolates were not found outside the patches. The weakly virulent isolates were present at much lower frequencies than the highly virulent and intermediately virulent forms within the patches. The frequency of occurrence of binucleate isolates did not vary significantly among the locations sampled. None of the multinucleate isolates contained plasmids. Some of the isolates contained a prominent single dsRNA species and one or more minor dsRNA species. The distribution of these dsRNAs was not correlated with pathogenicity.

Evidence of stability of pectic zymogram groups within Rhizoctonia solani AG-8

Within R. solani AG-8, the cause of rhizoctonia bare patch disease of cereals, legumes and pasture, five zymogram groups have been identified. Evidence is presented that demonstrates the five groups (currently designated ZG1-1, ZG1-2, ZG1-3, ZG1-4 and ZG2) are very stable. These groups have a wide distribution in the cereal growing regions of southern Australia and some have been detected in the Pacific Northwest of the U.S.A. We propose that ZG2 be renumbered as ZG1-5 to create a standard numbering system for zymogram groups within AG-8.

Mycelial Interactions and the Potential Use of Tuft Formation in Characterizing Rhizoctonia solani Isolates Infecting Cereals

Field isolates of Rhizoctonia solani anastomosis group (AG) 8, the most important causal pathogen of cereal bare-patch disease, were paired with each other and with tester strains of other AGs on potato-dextrose agar amended with charcoal (PDCA) to investigate mycelial interactions. Pairings among AG 8 field isolates produced compatible interactions of either tuft or merging reactions. Tufts formed between all paired field isolates from different pectic zymogram groups (ZGs) within AG 8, but pairings between genetically identical isolates showed merging reactions. Pairings of AG 8 field isolates with the tester strains of the other AGs led to incompatible interactions varying from merging line to barrage reactions. As formation of a tuft indicates that the paired isolates are able to anastomose and to form viable heterokaryons, the testing of mycelial interaction types, highlighted by tuft formation, may be used as a rapid procedure to characterise field isolates of R. solani obtained from cereals.

Heterokaryon formation with homokaryons derived from protoplasts of Rhizoctonia solani anastomosis group eight

Homokaryons were successfully recovered by regenerating protoplasts prepared from vegetative hyphae of field isolates of Rhizoctonia solani anastomosis group (AG) 8, the causal pathogen of bare-patch disease of cereals. A mating type incompatibility system, which is similar to that reported in AG 1 and AG 4, was demonstrated in AG 8. All homokaryons obtained in AG 8 were able to form tufts with their parent isolates and other heterokaryotic field isolates of AG 8 tested. Heterokaryons were readily recovered from tufts of pairing of certain homokaryon combinations. The synthesized heterokaryons formed tufts with both of the contributing homokaryons. The majority of hyphal tip cultures isolated from tufts resembled one of the contributing homokaryons. These nonheterokaryotic hyphae in tufts are attributed to transient heterokaryon effects.

Interactions between organic-matter decomposition level and soilborne disease severity

Literature reviewed shows that no-tillage small-grain production increases Rhizoctonia bare-patch disease in arid climates. The type and amount of residues, the method of their incorporation and the level of decomposition at the time of planting appear to affect disease severity. Partially decomposed residues of the previous season confound interpretation of the current season's input. Interactions between organic-matter decomposition level and disease severity in soil systems, therefore, are difficult to resolve. Organic-matter transformations that occur during decomposition of wastes in compost piles are similar to those in soil. This process, therefore, was tested as a model to determine effects of organic-matter decomposition level in soil systems. Both Rhizoctonia solani and its antagonist Trichoderma hamatum developed to high population levels in a substrate amended with fresh organic matter. However, high levels of disease also developed. A negative relationship was found between increasing decomposition levels of the organic matter (compost age) and population development of both the pathogen and its antagonist, as well as with disease severity. On the other hand, addition of high levels of cellulose to this disease-suppressive substrate amended with a mature compost, in amounts equivalent to levels of cellulosic substances present on the surface of soil in no-tillage small-grain production, stimulated population development of both the pathogen and the antagonist and negated the suppressive effect. We conclude that the composting process can be used to examine interactions between specific crop-residue decomposition levels, the host, the pathogen and its biocontrol agents.

Effect of interaction of Rhizoctonia spp. with other fungi from cereal bare patches on root rot of wheat

Rhizoctonia solani (Ag 8), binucleate Rhizoctonia sp. (Ag C), Fusarium graminearum, Bipolaris sorokiniana, Waitea circinata, Mortierella sp. and Pythium irregulare, from bare patches of wheat and barley were evaluated as root pathogens of wheat. Combinations of these fungi caused more severe root disease than the same fungi used singly. A complex of these fungi rather than any one pathogen is proposed as the cause of the bare patch disease of cereals.

Identity and pathogenicity of Rhizoctonia spp. associated with bare patch disease of cereals at a field site in Western Australia

Roots of seedlings of wheat and barley affected by bare patch disease at a field site in Western Australia were assessed for root damage and plated to isolate fungi. The patches were variable in shape and size and had the most severely affected plants in the centre. Of the 165 isolates ofRhizoctonia spp. obtained, 90% were multinucleate and 10% binucleate, the former being predominant in the plants at the centre of the patch. The relative frequency of binucleate isolates increased with proximity to the periphery. The increase in activity of avirulent binucleate isolates towards the periphery of the patch may be related to the sharp and abrupt edging of the patch. A variety of other species of fungi such asFusarium spp.,Mortierella spp.,Bipolaris sorokiniana, Pythium sp. andTrichoderma sp. were encountered within the patches. The multinucleate isolates belonging to anastomosis groups (Ag) 2–1, 2–2 and 8 (Thanatephorus cucumeris) were most pathogenic to wheat. The binucleate isolates of Ag C, D, E, and K (Ceratobasidium sp.) were less pathogenic. It is suggested that the bare patch disease is caused by a complex of root rot fungi composed of one or more anastomosis groups ofRhizoctonia spp. and other associated fungi. Van kiemplanten van tarwe en gerst, afkomstig van een met kale-plekkenziekte besmet perceel in West Australie werd de mate van wortelbeschadiging bepaald en werden schimmels uit de wortels geisoleerd. De plekken waren verschillend van vorm en afmeting; de zwaarst aangetaste planten werden in het centrum ervan aangetroffen. Van de 165 verkregen isolaten vanRhizoctonia spp. was 90% meerkernig en 10% tweekernig. De meerkernige overheersten in de centra van de plekken. Relatief gezien nam het aantal tweekernige isolaten toe naarmate de herkomst dichter bij de periferie van de plekken was. De scherpe begrenzing van de ziekte aan de randen van de plekken zou in verband kunnen staan met het toenemen van de activiteit van de avirulente tweekernige isolaten in de nabijheid van de periferie van de plekken. Een aantal andere schimmels, zoalsFusarium spp.,Mortierella spp.,Bipolaris sorokiniana, Pythium sp. enTrichoderma sp. werd eveneens in de plekken aangetroffen. De meerkernige isolaten die tot de anastomosegroepen Ag 2–1, 2–2 en 8 (Thanatephorus cucumeris) behoren, waren voor tarwe het meest pathogeen. De tweekernige isolaten van de anastomosegroepen Ag C, D, E en K (Ceratobasidium sp.) waren minder pathogeen. Gesuggereerd wordt, dat de kale-plekkenziekte veroorzaakt wordt door een complex van verschillende wortelschimmels, die behoren tot een of meer anastomosegroepen vanR. solani en andere daarmee geassocieerde schimmels.

Bare-patch disease and associated problems in subterranean clover pastures in South Australia.

Three diseases new to subterranean clover pastures have been observed in the Coonalpyn Downs, South Australia. Rhizoctonia solani is shown to be the cause of "bare patch" and Meloidogyne marioni of "root-knot patch". The third disease, "unthriftiness", is a complex problem. There is some evidence that Corticium praticola may be associated as a parasite, but it appears that there are other contributing factors. The implications of widespread establishment of these diseases are discussed and suggestions for control are made.