Thanatephorus cucumeris (many names, depending on host)

General

Early infection gives rise to seed decay and pre- and post-emergence damping-off. Later infection causes stem canker, wirestem, eyespot and other diseases which result from the decay of the stem cortex and may be accompanied by stunting, yellowing and leaf roll symptoms. Other diseases include rot of organs in contact with the soil, web, leaf and thread blights, fruit rot, root rot and storage rots and blemishes. Sites susceptible to infection are generally more restricted with increasing maturity of the host (Mordue, 1974).

Potato

Symptoms on potato are typically observed from 3 to 4 weeks after planting when emergence is seen to be patchy. Deep, dark-brown lesions develop on the underground stems and stolons (stem and stolon canker, respectively). Lesions merge to form necrotic collars (a process known as girdling), which then leads to the stem/stolon being severed (pruned).  Pruning may occur on primary and secondary stems leading to an overall delay in emergence. Diseased plants are unable to compete with healthy neighbouring plants and are consequently lower yielding. Banville (1989) calculated that yield loss from stem and stolon canker could be up to 30%. Damaged stolons can result in a reduction of the marketable ware fraction due to an increase in larger tubers from the remaining primary stolons and an increase in undersized tubers from lateral stolons. Other symptoms such as tuber greening and netted scab are associated with stolon canker. During crop senescence volatiles are released from the decaying underground stems to stimulate the development of sclerotia on the daughter tubers (black scurf). Black scurf significantly reduces the value of potatoes sold to supermarkets and downgrades the quality of seed potatoes. Muzhinji et al. (2014) report elephant hide and cracking on potato tubers from South Africa.

Rice

Sheath blight symptoms on rice are usually observed when the crop reaches its full vegetative growth: they can be observed at the stages of maximum tillering or stem elongation. The disease produces lesions on the leaf sheaths and the leaf blades. Typical lesions are at first ellipsoid or ovoid, somewhat irregular, greenish-grey, varying over 1-3 cm long (Ou, 1985). The centre of the lesion becomes greyish-white, with a brown margin. The size and colour of the lesions vary depending upon the age of the lesion and environmental conditions. Coalescence of several lesions results in blighting of part or whole leaves. Sclerotia are formed superficially on or near lesions on leaves, stems and seeds. Sclerotia are loosely attached and easily dislodged from the plant when mature. Care should be taken in field diagnosis of sheath blight, as sheath spot (Rhizoctonia oryzae) and aggravated sheath spot (R. oryzae-sativae) also occur in parts of Asia and may exhibit similar lesions.

For a discussion of anastomosis groupings, see Notes on Taxonomy.

Introduction

Very many scientific publications have evaluated methods of controlling Rhizoctonia diseases, using a wide variety of approaches. In South-East Asia these have been principally concerned with rice sheath blight, but some examples of control approaches for Rhizoctonia diseases of other crops are also described below. Note that these are illustrative examples rather than a definitive guide to control methods for all crops affected.

For control of seedling diseases, control can be obtained through heat or chemical pasteurization of the planting medium in nurseries, and fungicide seed treatment can reduce infection in field-sown crops. Soil fumigation (using broad-spectrum chemical fumigants) may be feasible for high-value crops.

Cultural Control

In China, application of calcium superphosphate may reduce disease severity. Draining fields at an appropriate time (e.g. at maximum tiller or panicle initiation stages) can also control the disease (CAAS, 1986).

In Indonesia, potassium application reduced disease severity. Row planting, in contrast to the common square planting, is expected to decrease the relative humidity and temperature below the crop canopy and increase evapotranspiration and sunlight penetration: these conditions are unfavourable to sheath blight.

In Vietnam, field sanitation by removing weeds, well-balanced NPK fertilizer application, and low planting density may be used to reduce disease severity.

Soil solarization offers another prospect for control in areas with high levels of sunshine and has been used to control Rhizoctonia root rots of Gerbera (Kaewruang et al., 1989).

Brassica plants with high glucosinolate contents can be exploited in the management of soilborne pests and diseases using a process known as biofumigation. When the foliage of these plants is macerated and mixed with soil the glucosinolates become hydrolysed to release a variety of toxic and volatile compounds such as isothiocynates. Research has shown that Rhizoctonia diseases can be reduced using this method (Larkin and Griffin, 2007).  

Sheath blight severity was reduced by the application of silicon in the USA (Winslow, 1992; Deren et al., 1994).

The severity of stem canker of potatoes may be reduced by planting during warmer conditions, or by using pre-sprouted seed tubers. Both practices help promote rapid emergence and disease escape.

Host-Plant Resistance

Many disease-resistance trials have been conducted to identify resistance to sheath blight in rice; however, no entry demonstrated significant resistance to sheath blight (Raina et al., 1992).

Transgenic tobacco plants show enhanced resistance to Rhizoctonia solani. As there is no effective source of sheath blight resistance in rice, transgenic mechanisms could provide a route to enhanced control of this pathogen.

R. solani attacks many parts of a wide variety of plants. The fungal cell wall depends on chitin for mechanical strength; chitinase treatment of fungal mycelia leads to bursting of cells at the growing hyphal tip. Tobacco seedlings that expressed elevated levels of chitinase after transformation with a gene from bean that codes for chitinase showed better survival rates in soil heavily infested with Rhizoctonia solani than did control plants. The extent of disease resistance observed in the 'chitinase' tobacco plants varied with the amount of fungal inoculum, a property that is characteristic of quantitative resistance.

Biological Control

This control method is still at an experimental level although there are a few products that are commercially available. Trichoderma spp. were considered 'promising' against R. solani (Gokulapalan and Nair, 1992).

Bacteria such as Pseudomonas spp, Streptomyces spp. and Bacillus subtilis have all shown potential against R. solani diseases.

The integration of fungal antagonists and organic amendments has also given field control of rice sheath blight (Baby and Manibhushanrao, 1993).

Introduction

T. cucumeris is a pathogen with a worldwide distribution and in Japan alone T. cucumeris is reported to infect 35 orders, 52 families, 125 genera and 142 species of plant (Ogoshi, 1996). Yield and economic losses caused by T. cucumeris have not been determined in the majority of crops and environments. The most comprehensive estimates of losses are available for Rhizoctonia bare patch of wheat in Australia, sheath blight of rice in Asia and cotton seedling disease in the USA. The remainder of the reports cited are for only a few crops in selected regions and in selected years.

Brassicas

Under conditions favourable for disease, T. cucumeris is involved in pre and post emergence damping off, root rots and leaf blight of brassicas. Populations of T. cucumeris isolated from diseased plants normally include isolates of AG 2-1, 2-2, 4 and 9. Losses due to root rot are generally due to a pathogen complex involving Fusarium, Pythium and Rhizoctonia, with Rhizoctonia being the dominant pathogen (Calman et al., 1986; Calman and Tewari, 1987; Gugel et al., 1987). The severity of rapeseed root rot varies between the Canadian provinces. Incidence and severity are low in Manitoba and Saskatchewan (Rimmer and Platford, 1982) but are high in Alberta where losses of stand vary from slight to nearly complete (Sippell et al., 1985a, b) with average losses in 1983 and 1984 estimated to be 30% (Sippell et al., 1985b).

In the UK, a survey of field-grown Brassica oleracea crops showed that out of 1300 plants sampled and tested with real time PCR, 116 were positive for Rhizoctonia solani, with AG2-1 and AG4 HGII making up the majority of the isolates recovered. Lamprecht et al. (2011) found that the survival of rape (Brassica rapa) seedlings in soil infested by R. solani AG 2-1 was between 51.3-68.7%.  Recently an isolate of AG10 was found to cause root rot on rape in Washington State in the USA (Schroeder and Paulitz, 2012).

Cereals

T. cucumeris AG-2-2 causes banded leaf and sheath blight of maize. In India, the loss in grain yield in one study of ten cultivars was estimated to range from 24 to 32% (Lal et al., 1980) with the overall loss for the whole of India estimated to be 1% of the total grain yield of maize (Payak and Sharma, 1985). China is the only other country in which losses have been estimated for the disease. In Guanxi Province in south China, the yield loss under natural conditions was 88 and 58% in two maize hybrids (Zhang et al., unpublished) and in Jiangsu Province in eastern China, yield losses ranged from 5.6 to 59.6%.

T. cucumeris AG 8 causes bare patch disease in cereals in Australia and the Pacific North-West of the USA. Rhizoctonia bare patch disease in cereals in Australia varies with season and has changed with changes in management practices. Early work in the 1930s showed losses of 8-35% in individual wheat fields with yields from inside patches reduced by 60-86% (Hynes, 1937). MacNish, in the mid 1980s, demonstrated that wheat yields within patches were 8% of that outside patches and that patches occupied 26% of the field (MacNish, 1986).

In 1996-97, Australia produced 23 million tonnes of wheat, 6.7 million tonnes of barley and 1.7 million tonnes of oats. Average annual losses in wheat caused by T. cucumeris AG 8 in southern New South Wales are 0.2%, in Victoria 3.8%, in South Australia 3.7% and in Western Australia 0.4%, with a total loss of 1.2% of the Australian wheat crop annually. These yield losses result in an economic loss of A$0.8 million in southern New South Wales, A$13.9 million in Victoria, A$16.4 million in South Australia and A$4.0 million in Western Australia for a total loss for Australia of A$35.2 million annually (ABARE, 1999). No published estimates are available for losses in barley and oats which can suffer losses as great or greater than wheat.

Economic losses due to bare patch in severely affected wheat in the Pacific North-West of the USA are similar to average losses recorded in the Australian states of Victoria and South Australia. In 1984, losses in wheat were up to 3% (US$25/ha) at Hermiston, Oregon (Pumphrey et al., 1987) and in 1987 losses in wheat were 4% (US$37/ha) and in barley 8% (US$22/ha) in Helix, Oregon (Smiley et al., 1989). In Indiana, early spring root and crown rot disease caused by a complex of snow mould (Typhula), Rhizoctonia root rot and Rhizoctonia sharp eyespot resulted in estimated yield losses of 5% in 1972, 5% in 1982, 3% in 1984 and 0.5% in 1987 (Patterson et al., 1990).

In both the USA and Australia, loss estimates for bare patch disease of wheat were based on the percentage of crop displaying patches and the comparative yields inside and outside patches. This may underestimate losses, as roots are damaged and plants are also stressed outside of the patches.

Cotton

As part of a disease complex with Pythium, Thielaviopsis and Fusarium, T. cucumeris AG 4 causes seedling disease of cotton which results in pre- and post-emergence damping off (Watkins, 1981). Surveys of 14 American cotton growing states in 1971, 1972 and 1973 showed that seedling diseases caused average losses of 4, 2.6 and 2.7%, respectively (USA, 1972, 1973, 1974). Between 1995 and 1999, there was an estimated average 3.1% loss in cotton production due to seedling diseases in the USA. In 1995, there was 3.82% or 830,000 bales lost, in 1996 there was 2.75% or 550,000 bales lost, in 1997 there was 3.54% or 635,000 bales lost, in 1998 there was 2.78% or 440,000 bales lost, and in 1999 there was 2.78% or 550,000 bales lost. These disease losses indicate little change in disease severity over the last 3 decades, and place seedling diseases as second only to nematodes as the most important disease of cotton. Losses varied between states between the years 1995 and 1999, with Alabama, Tennessee and North Carolina consistently recording the highest yield losses due to seedling diseases (6.5, 6.5 and 4.5%, respectively) and Arizona, New Mexico and Oklahoma recording the lowest (2, 0.8 and 1.2%, respectively) (Blasingame, 1996, 1997, 1998, 1999, 2000).

In New South Wales, Australia, the largest cotton-growing state, seedling establishment as a percentage of seeds planted has been measured for the 11 seasons between 1989/1990 and 1999/2000. Seedling establishment is a function of seedling disease caused by Pythium and T. cucumeris, seed viability and soil insect damage. There has been a gradual decline in seedling establishment losses between the 1989/1990 and 1999/2000 seasons with an average of 33.1% for the 1989/1990 to 1993/1994 seasons to an average of 22.4% for the 1995/1996 to 1999/2000 seasons (S Allen, Cotton Seed Distributors, Narrabri, NSW, Australia, personal communication, 2000).

Pasture and Forage

T. cucumeris is one of the four most damaging pathogens of pasture legumes in tropical American lowlands. The pathogen is the cause of Rhizoctonia foliar blight which causes annual losses of dry matter of 10-30% and up to 50% and greatly reduces seedling survival in Centrosema brasilianum in Colombia (Lenne et al., 1985, 1989). Rhizoctonia foliar blight is widely distributed throughout tropical America and has been recorded in eight other countries where it is most damaging in humid areas (Lenne et al., 1985,1989).

Siratro (Macroptilium atropurpureum) is another tropical legume affected by foliar blight caused by T. cucumeris. In naturally infected plots in Florida, USA, foliage yield was reduced by 80% and stem yield by 50% compared with the best fungicide treatment which almost completely eliminated defoliation caused by the disease.

Groundnuts

Rhizoctonia damage in groundnuts can be due to the pathogen operating alone or as a disease complex with other pathogens such as Pythium, Rhizopus, Fusarium and Aspergillus. T. cucumeris can attack roots, stems, pegs, pods and leaves of groundnut plants. Disease severity varies throughout the season. A survey of fields in the south-eastern USA showed that in September, 28% of fields had an average of more than 50% of the taproot cankered, but in October, 77% had disease severities of less than 25% and none more than 50% (Turner and Backman, 1988). In Georgia, USA, losses in 1988 were estimated to be US$44 million (Thompson, 1988). During 1990-1993 the estimated annual loss caused by Rhizoctonia damage to groundnuts in Georgia was $17.4 million (Hadden, 1990; Bertrand, 1991, 1992, 1993). Since 1994, the accuracy of loss estimates has increased due to the availability of fungicides effective against Rhizoctonia and yield losses have been estimated at $8.3 million annually in Georgia in the period from 1994 to 1999 (Bertrand, 1994, 1995, 1996, 1997; Williams-Woodward 1998, 1999).

Potato

T. cucumeris causes lesions on potato sprouts, roots and stolons and affects quality by producing sclerotia on tubers. The dominant anastomosis group is AG 3 but AG 4 and AG 5 may also be associated with the disease. T. cucumeris attacks potatoes worldwide with losses in potato yield as high as 30% (Logan and Cooke, 1984) but commonly between 10 and 15% (Little et al., 1988; Banville et al., 1989; Carling et al., 1989; Read et al., 1989). The quality of tubers is also affected and adds to economic losses. A study in Alaska demonstrated a loss of 19% of total yield but 35% of yield of US-1 quality tubers due to T. cucumeris (Carling et al., 1989). Similarly, a study in Canada over 3 years showed that total yield was reduced by 9 and 16% for two different potato cultivars due to T. cucumeris, but marketable yield for those cultivars was reduced by 21 and 26%, and numbers of size A tubers fell by 30 and 31%, respectively (Banville et al., 1989).

A field experiment investigating the effect of different anastomosis groups of R. solani on potato revealed that isolates of AG2-1, AG3PT, AG5 and AG8 could all cause a significant reduction in tuber weight (Woodhall et al., 2008). Inoculation with R. solani AG8 resulted in the highest reduction in tuber weight with an average weight loss of 23% when compared to the untreated control. Interactions with potato cyst nematodes (Globodera spp.) have been shown to increase the incidence of stolon infections (Back et al., 2006; Bhattarai et al., 2010).

Pulses

T. cucumeris causes four diseases in beans: damping off and seedling blight often caused by a complex of pathogens; root rot also caused by a pathogen complex; web blight of aerial parts of the plant; and finally postharvest seed rots.

In North America, Rhizoctonia root rot is an important disease of soyabeans. In Iowa, USA, a disease survey carried out in the central counties in 1967, a particularly bad year for Rhizoctonia root rot due to the wet season, showed that 12% of fields were affected by as much as 50%. No estimates were made for total yield loss (Tachibana, 1968). Farmers fields in the San Juan basin of Colorado, where 55,000 ha of Pinto beans were grown, were sampled in 1971 and 1972. No fields were free from disease, and moderate or greater disease was found in 45% of fields in 1971, and 59% of fields in 1972. An 84% reduction of yield was attributed to the disease in 1971 (Keenan et al., 1974). A root disease complex of beans also affects the semi-arid part of the state of Durango in Mexico in which T. cucumeris is a major component. In 1981, the pathogen complex caused a 36% loss of grain yield across 19 sites (Sanchez Anguianao and Cardenas Alonso, 1988). In contrast, in the North Platte Valley of Western Nebraska, a 3 year survey of Pinto and Great Northern beans beginning in 1969 determined that, based on isolation frequency, T. cucumeris was only a minor, early contributor to a root disease problem that caused losses of 13%. However, due to the often sequential nature of colonisation of plant tissue by pathogens in disease complexes, isolation frequency may underestimate the contribution of Rhizoctonia to disease loss.

In South America in 1993, a survey of bean diseases was undertaken in three regions of El Salvador. Despite 30% of farmers in the survey using chemical control for web blight, the disease incidence in 1993 was 26%, causing average losses of 13 kg/ha and resulting in an economic loss of US$7.1 million annually (Choto et al., 1993). In 1993 in Honduras in a similar study, 19% of the surveyed bean area planted was damaged by web blight but only 6% of farmers used chemical control measures (Gamero et al., 1993).

Australia produced 1.5 million tonnes of lupin seed in 1996-1997, 80% in the state of Western Australia (ABARE, 1999). In the shires of Dalwallinu, Wongan-Ballidu and Moora which are in the main lupin growing districts, annual grain losses of approximately 2-5% and occasionally 15% are experienced due to Rhizoctonia bare patch. Losses due to Rhizoctonia hypocotyl rot, another form of lupin disease, were not provided, but were considered to be more than that due to Rhizoctonia bare patch (Sweetingham, 1986).

Rice

T. cucumeris AG 1-IA causes sheath blight, a major disease of rice worldwide.

Between the years of 1985 and 1990, 47% of the total area under rice cultivation in China was affected by sheath blight, and the disease has been estimated to cause yield losses exceeding 200 million kg per annum. In Malaysia, 15-20% of the total area planted to rice was infected with sheath blight disease. The yield loss in the 1993 wet season was 17-25%. Sheath blight became important as the area planted to direct seeding increased to 80%. The intensive use of nitrogen fertiliser also contributed to the importance of the disease (Statistics from IRRI, 1993).

Sheath blight intensity in Thailand was very high in 1988 as a result of heavy rains and possibly also due to the wide areas planted with the susceptible cultivar RD23 (Statistics from IRRI, 1993).

Throughout Vietnam, the area with 15-30% yield loss due to sheath blight, increased almost tenfold in 5 years from 21,000 ha in 1985 to about 200,000 ha in 1990 and 1991 with 4000 ha and 127 ha suffering from 100% yield loss in 1990 and 1991, respectively. Sheath blight is second in importance to rice blast disease (Magnaporthe grisea) in Vietnam (Statistics from IRRI, 1993).

Losses in rice in the USA are high. The disease is cited as the most important rice disease in the southern rice producing areas of the USA in the mid 1970s to mid 1980s with yield losses as large as 50% (Lee and Rush, 1983).

Sugarbeet

T. cucumeris disease of sugarbeet is caused predominantly by AG 2-2 although AG 1, AG 2-1, AG 4 and AG 5 have also been recorded. T. cucumeris produces a seedling disease involving a complex of pathogens which also includes Aphanomyces and Pythium. Mature plant diseases involving crown and root rot or crater rot are also caused by T. cucumeris. Whitney and Duffus (1986) estimated that mature plant losses average 2% of yield but that losses vary from 0 to 50%. In the Netherlands, the disease occurs on 80% of the sandy soils in the east and south-east of the country with 10% of these soils associated with severe crop losses (Schneider, 1999). It has been estimated that 2% of sugarbeets in Austria are diseased by Rhizoctonia, 1% in Belgium, 5% in France, 1% in Germany, 5% in Greece, 2% in Italy, 13% in the Netherlands and 8% in Spain (Schneider, 1999). At Epen in the Netherlands in 1996, resistant varieties from the USA grown alongside the local susceptible variety Auris in a naturally infested farmers field showed nearly an 80% loss in root weight (Schneider, 1999). Actual losses are likely to be even greater as the most effective resistant sugarbeet cultivars only reduced disease severity by an average of about 50% at three locations in 1998 (Schneider, 1999).

Sugarbeet quality appears to be related to the severity of R. solani infections and problems may arise at sugarbeet refineries (Bruhns et al., 2004).

Another disease of sugarbeet caused by T. cucumeris is foliar blight, but its economic importance has not been determined.

Tobacco

T. cucumeris in a complex with Pythium causes damping-off disease of tobacco seedlings in greenhouses, with losses in Ontario, Canada, in 1972 of 5-10% and in 1973 of 3-5%. On plants in the field, T. cucumeris causes sore-shin disease which in 1972 caused losses of about Can$ 500,000 and replanting costs of a further Can$ 500,000, but in 1973, a drier year, losses were Can$ 400,000 with little replanting taking place. The mean yield loss due to disease in Ontario over the 2 years was 0.3% (Gayed and Watson, 1975). In the south-eastern USA, T. cucumeris sometimes causes serious losses in tobacco through target spot lesions, damping off and sore-shin (Csinos and Stephenson, 1999).

Vegetables and Herbs

Rhizoctonia in vegetables causes damping off, leaf blights, root and stem rots and fruit rots.

In Manicore county in Brazil, watermelon leaf blight caused by T. cucumeris caused yield losses of 20-50% with 100% of fields affected (Santos and Boiteux, 1994).

Fruit rot of cucumber caused by T. cucumeris is among the most serious disease in the south and south-eastern USA. Yield losses can be up to 80% with average annual losses of 7-9% which, in the mid 1970s, was estimated to be worth US$4-5 million (Sumner and Smittle, 1976; Lewis and Papavizas, 1980; Jenkins and Averre, 1981).

In naturally infected plots in commercial fields with a history of bottom rot of lettuce in Berlin, Wisconsin, USA, yield losses varied with the date of planting and between seasons. In 1982 in the May planting, application of the best fungicide treatment reduced the disease index from 83 to 38% and increased yield from 5900 heads per acre in the untreated control to 19,800 heads per acre in the fungicide treated plots, indicating at least a 70% loss in untreated plots. Similar losses of 60-80% have been recorded in heavily infected plots of lettuce in the Isere Valley, France (Camporota et al., 1986). Losses in glasshouse grown lettuce in the Netherlands due to bottom rot disease caused by Sclerotinia, Pythium and Rhizoctonia were, on average, 17% without treatment and 5% with fungicide treatment (Kooistra, 1983).

Geranium, an important essential oil crop in India, suffers a major root rot and wilt problem caused by T. cucumeris in southern India. Field studies of naturally infected plants in 1987-1990 showed mortality rates of 100, 52 and 50% after 420 days in three different clones planted in summer. In the same experiment, mortality rates of 100, 31 and 32% were measured after 420 days for the same clones planted in winter (Kalra et al., 1992).