Rhizoctonia Introduction There are various species of the fungus genus Rhizoctonia, of which the species Rhizoctonia solani and Rhizoctonia violacea can affect sugar beet. This pamphlet deals with the species Rhizoctonia solani. Information about Rhizoctonia violacea is added at the end. Rhizoctonia solani is a fungus found world-wide which attacks a great number of crops as a soil-born pathogen. Different origins of Rhizoctonia solani can cause different diseases on sugar beet. According to the latest state of knowledge Rhizoctonia solani is of secondary importance as a pathogen in the complex of "Strangles" ("Black Leg", "Damping-off"); but this will not be further considered here. The most important is a disease called Late Root Rot. The aim of this pamphlet is to provide more information about the disease Late Root Rot and precautions to reduce the damage. Origin - History - Spread of the Disease - Economic Importance In 1858, Julius Kühn, the founder of agricultural phyto-medicine, called a fungus Rhizoctonia solani, the root killer, which he had isolated from infected potatoes. Rhizoctonia solani is a world-wide fungus which is found in almost all soils. Though originally not appearing in cultivated soils, the fungus could settle in many agricultural and horticultural crops. For a long time the high variability of the fungus made a clear link to infected host plants difficult. Meanwhile, numerous tests have shown that there are various strains of Rhizoctonia solani which are classified in so-called anastomosis groups (AG). Depending on the affiliation to a certain anastomosis group, Rhizoctonia solani can attack different crops, as for instance sugar beet, potatoes, cereals, rape, maize, soyabean, and others. There are two anastomosis groups affecting sugar beet: The AG 2-2 is the sole pathogen of Late Root Rot. The AG 4 is of importance only in the pathogenic group "Strangles" and as pathogen of the Late Rot of Leaves in the US and Japan. The fungus has been found for many years in the United States, Chile, Japan and Southern Europe as an economically relevant pathogenic agent, it has gained in importance in Germany and other countries of Central Europe as a sugar beet disease only since the early 90s. The area infected with Late Root Rot in the EU has been assessed at about 70,000 ha, and it is expected that the importance of this disease is increasing.
Fig. 1 Areas affected by Late Root Rot in European countries Often Rhizoctonia solani causes severe damage to individual plants. The losses suffered in root yield are most variable, depending on the time of infection and the extent of the infected area, they may range between 2 and more than 50 %. Tests have shown that the sugar content is reduced by more than 60 % with a severe infection with Rhizoctonia solani. Besides poor storage quality the processing quality of affected beet is highly impaired. Depending on the severity of attack picking out rotten beet may become necessary. 20 sugar content (%) 15 10 5 0 without low moderate heavy infection Effects of Late Root Rot on the root yield and quality of sugar beet (according to Büttner & Bürcky 1999)
potassium (mmol/1000g) 60 40 20 0 without low moderate heavy infection Effects of Late Root Rot on potassium content of sugar beet (according to Büttner & Bürcky 1999) 15 sodium (mmol/1000g) 10 5 0 without low moderate heavy infection Effects of Late Root Rot on the sodium content of sugar beet (according to Büttner & Bürcky 1999) 16 alpha-amino-n (mmol/1000g) 12 8 4 0 without low moderate heavy infection Effects of Late Root Rot on the alpha-amino-n content of sugar beet (according to Büttner & Bürcky 1999)
Symptoms Initial symptoms of Late Root Rot can occur as soon as the closing of the rows, but often an infection is visible only in late summer or autumn. In small, mostly clearly marked patches firstly a sudden wilting or chlorosis of the leaves takes place from outside inwards and the infected beet often show stunted growth. In the upper third of the root body, often immediately below the soil surface, greyish-brown to black rotten spots or fissures become visible first of all only on the root surface. In the advanced stage, these rotten spots merge into one another. A cross-section through the root body shows a clear division between healthy and infected tissue. Late Root Rot starts just below the soil surface and spreads over the whole root with growing disease development. With a highly advanced rot the wilted leaves are lying in a circle on the soil round the beet; the beet still tries to form new leaves before it dies. In the affected patches often all stages of infection are found together. Due to the infection of diseased beet with fungal and/or bacterial secondary pathogens, dry or wet rots of the root are likely to occur. Plant population gaps caused by Late Root Rot Typ. symptom of infection on a severely attacked sugar beet plant -the dead leaves are lying in a circle on the soil round the beet
Comparison between a healthy and an infected root - progressive discolouration of the root body from outside inwards Symptom of Late Root Rot in the final stage Possible Confusions and Methods of Detection In spite of increased occurrence of the disease in the last few years, rotten, brownish to black looking beet are not necessarily infected by Rhizoctonia solani. Possible confusion is likely especially with other pathogens attacking the root body. An infection with Rhizoctonia violacea, a closely related fungus, starts infection - unlike Rhizoctonia solani - in the lower area of the root. The attacked tissue is coated with a reddish to dark-violet coloured mycelium. Phoma Root Rot (Phoma betae) causes a dry rot which likewise starts at the upper part of the root body. The infected tissue becomes dark-brown to black and can split open. Often an additional attack of the leaves (Phoma leaf spots) can be observed. Phytophtora Wet Rot (Phytophtora megasperma) is responsible for a root rot which - unlike Rhizoctonia solani - starts at the root tip. Here the foliage yellows and wilts and the plant is retarded in growth. Clamp Rot is caused by a group of pathogens, various fungi may be involved (among others Fusarium
spp., Penicillium spp., Aspergillus spp., Botrytis cinerea). In the crown area of harvested beet a dense fungus coat develops which can be differently coloured depending on the prevailing pathogen. Similar to Rhizoctonia solani Girdle Scab occurs mainly on badly structured soils with a high or low ph value depending on the causal agent. In wet summers the infection with Girdle Scab is generally widespread. Unlike Rhizoctonia solani parts of the root are scurfy and brownish coloured in a band round the beet, partly with strangles and protruding deformations on the root surface. There is no symptom of rot, the foliage keeps unaffected until harvest. Girdle Scab, a disease which may cause black rotten roots even in clamps Stem and Bulb Eelworm (Ditylenchus dipsaci) likewise cause scurfy, chapped spots which are turning black later on. In the further course, secondary infections with fungi, bacteria and other nematodes cause rots (Crown Rot) which reach deeply into the tissue. Root rot in the crown area caused by stem and bulb eelworm (Ditylenchus dipsaci)
Boron deficiency is responsible for Heart and Dry Rot. Unlike Late Root Rot, this rot begins in the area of heart leaves which become black and die. Older leaves show fine fissures on the surface and growth of callus in the petiol area, before they wilt and likewise die. Boron deficiency, this rot starts in the heart leaves area In particular, secondary infections by fungi, bacteria and nematodes make an accurate diagnosis more difficult. Even if the described symptoms exist and the other prerequisites of an attack by Rhizoctonia solani are given, a reliable diagnosis is guaranteed only by the detection of the pathogen in the laboratory. As standard method in the laboratory plant material from the transition area between infected and healthy tissue is laid out in Petri dishes on specific media for Rhizoctonia solani. By means of typically morphologic characteristics of the growing mycelium the pathogen can be positively determined under the microscope. More modern methods work on the basis of specific laboratory methods (ELISA) or with the aid of molecular-biological methods (PCR). Biology and Epidemiology of the Pathogen Rhizoctonia solani belongs to the class of Fungi imperfecti and is the asexual form of the main perithecial form Thanatephorus cucumeris. Numerous isolates of Rhizoctonia solani have been described. A socalled compatibility test examines whether hyphae of two isolates are capable of coalescence. Based on compatibility, the isolates are classified in so-called anastomosis groups (AG). These differ from each other mainly regarding their host plants and the temperature at the time of infection. So far, 12 anastomosis groups have been described world-wide; through new molecular-biological techniques (PCR) and biochemical methods a further differentiation is to be expected. Within the different anastomosis groups, subgroups exist which distinguish by a more or less narrow group of host plants. For the anastomosis group 2-2 which is important for sugar beet so far the subgroups AG 2-2IIIB and AG 2-2IV have been described, Late Root Rot being caused mainly by AG 2-2IIIB. Heavy, badly drained soils liable to flooding and a bad structure as well as warm weather conditions in spring and summer (>25 C) favour infections. In some countries, as for instance the Netherlands, the fungus even occurs on sandy soils with a high water table.
Rhizoctonia solani survives on organic substance in the soil, as sclerotia and/or as mycelium. Therefore, high amounts of organic matter have a favourable effect upon infection. Due to the high saprohytic potential of the fungus three years' survival as mycelium on organic matter is possible. The activity of the fungus starts with increasing soil temperature (>15 C). After having coated the surface of the root body or the petioles (soil contact) the hyphae directly penetrate the plant tissue with the aid of cell wall dissolving enzymes. Tests have shown that Rhizoctonia solani AG 2-2IIIB may attack both petioles and beet top and the root tissue as well. The lower sections of the pe-tioles are preferred entrance gates as well as the natural fissures in the beet top and lenticels at tap and lateral roots. In the plant tissue the fungus may spread between and within the cells. By an infection of the vascular bundle system the plant would be heavily damaged and/or dies off, as water and nutrient supply is impeded or even interrupted. The spread of the fungus through soil particles (e. g. soil cultivation equipment, harvesters) and also through wind or water erosion (drainage water, irrigation) is possible, but it is considered of minor epidemiological importance. Moreover it is assumed that Rhizoctonia solani is found in all soils and that an outbreak of the disease is triggered by external factors (crop rotation, weather, structural damage of the soils, water supply as for instance by irrigation). This explains the fact that in the last few years almost simultaneously a clear increase in infection has been observed in France, Germany, the Netherlands and other European countries, however it has not yet been identified in the UK as a Root Rot. In the UK a large proportion of the sugar beet is grown on light soils, where in the absence of irrigation, prolonged drought during the summer can cause severe wilting of healthy beet. The beet leaves consequently collapse onto the crown, very often at the point of contact fungal infection occurs. Measures Reducing the Damage As with other soil-borne pathogens of sugar beet the course of the disease and the extent of the damage are determined by the potential of infection in the soil as well as by climatic conditions and cultivation measures. Depending on the occurrence and on the time of infection by Rhizoctonia solani yield losses of more than 50 % may be the result. Present test results prove that with varying severity of the attack a clear reduction of sugar content and an increase of the potassium, sodium and alpha-amino-n levels may be the consequence (see above). The following soil and plant cultivation measures may reduce the damage: Crop Rotation The extension of crop rotation reduces the inoculum in the soil. Sugar beet in the rotation should not exceed one year in three. The inclusion of maize in the crop rotation is problematic, as Rhizoctonia solani (AG 2-2) can survive on maize straw. A further disadvantage is the often late harvest of maize and the inherent danger of damage to the soil structure. The cultivation of suitable intercropping is to be included in the crop rotation. Soil Preparation Good soil conditions are important, i. e. good drainage, correct ph etc. Avoid soil structure damage in the preceding crop and do not plough or cultivate in unsuitable conditions. In the case of severe infection an early harvesting date is to be preferred. Fertilisation The supply of nutrients should be adapted to the seasonal demands of sugar beet. The fungus can survive on organic material in the soil. Organic material with a close C/N ratio and low ph support the fungus. Plant Protection Measures
Spray applications of various active ingredients are able to reduce the attack. The problem is to apply the fungicide in sufficient concentration on the place of infection, so that the fungus growth can be definitely retarded. Seed treatments with a combination of antagonists and fungicides are being developed. The aim of this process is to control the early occurrence of Rhizoctonia solani with the fungicide portion and in addition to guarantee a long-standing protection by the placement of antagonistically acting bacteria in the rhizosphere. Choice of Varieties The choice of varieties is of vital importance. Since the introduction of new varieties that are more resistant or tolerant to Rhizoctonia solani the farmer is able to utilize high-yielding varieties in infected fields. Breeding for Resistance Resistance breeding occupies a prominent rôle at KWS. In many cases it is not possible nor economical to control diseases by plant protection chemicals. Therefore, KWS makes enormous efforts in the breeding of resistant varieties to supply the grower with suitable, high-performing varieties for his specific problem. So, in 1999 KWS could already market the first variety registered in Europe with resistance to Rhizoctonia solani. Source of resistance - where does resistance in sugar beet come from? The present registered or/and officially tested varieties with resistance to Rhizoctonia solani are mainly derived from American breeding material. The American original lines show a good resistance against Late Root Rot, but on the other hand they have only weak yield performance, below-average quality and low bolting resistance. The predominant target of breeding work therefore is to improve yield and quality by back-crossing the original lines with high-performing breeding strains and to increase bolting resistance. In addition, the search is on for new sources of resistance in wild beet. This work is still in its infancy. Until varieties are developed from this material, many years of breeding work will elapse. At every breeding stage, it has to be guaranteed that the new lines have not only been improved in their performance, but also that the resistance to Rhizoctonia solani has been preserved in its genotype. As resistance to Late Root Rot is transmitted by more than one single gene, part of the resistance may be lost within the back-crossing programs. Then the resulting lines are marked by a lower strength of resistance and they would thus offer only minor protection in the case of heavy attack. In order to avoid this and to guarantee a high resistance in the breeding material, intensive resistance tests are required. KWS conducts resistance tests on Rhizoctonia Root Rot in many countries world-wide. Resistance test - how can resistance differences be recognised? The resistance and tolerance test of breeding material and varieties is carried out today under natural conditions of infection or after artificial infection in the field. As root rot in the field mostly occurs only in patches and the severity of attack in different years is highly affected by environmental conditions, often several years of experiments are necessary in order to identify the best components for new varieties. For recording tolerance and resistance differences besides yield ratings under infection also counts of plant losses and field observation of the roots are carried out at harvest, where the degree of root rot is scored for every single beet. In a large trial field often several thousand beet must be examined individually to assess the extent of root rot. Breeding material testing therefore is rather expensive and the reason for the high development costs of new varieties. Tolerant varieties - a step towards protecting against Late Root Rot In the last few years in Europe tolerant varieties have been registered and grown firstly in France. In other countries, applications for registration have been made. The use of the first generation of varieties is recommended at present only for regions of severe attack. At high pressure of infection with Rhizoctonia solani it offers very good protection against root rot, but in
uninfected areas its yield is lower than achieved by other well-known standard varieties. This will improve with further breeding work. Due to the spread of Rhizoctonia root rot in areas with Rhizomania infection in the next few years varieties are being developed with a combined tolerance to Rhizoctonia and Rhizomania. The resistance of currently available varieties is exclusively against Late Root Rot and is hardly any value against seedling infection. Therefore, besides the use of tolerant varieties, sound crop rotation measures, good soil preparation and optimum sowing and field emergence conditions are decisive for guaranteeing a good protection of the beet over the whole growing period. Although fungal Root Rots are undoubtedly occurring with increasing frequency in the UK, their occurrence is unpredictable. This problem is recognised by KWS and an increasing amount of research is being invested in resistance breeding. In some European countries where there is a regular occurrence, sugar beet varieties resistant to Rhizoctonia solani are available, however at present there is a small yield penalty compared with current susceptible varieties in the absence of the infection. Summary and Outlook Rhizoctonia solani is a world-wide fungus which is present in almost all soils. Depending on the affiliation to an anastomosis group Rhizoctonia solani can attack different crops; the AG 2-2 causes Late Root Rot on sugar beet. The fungus survives in the form of sclerotia, as mycelium and/or on organic matter in the soil. Warm climates and high soil moisture favour attack by Rhizoctonia solani. The infection manifests itself from summer to autumn firstly in small patches. The beets wilt and show poor growth. Greyish-brown to black rotten patches and fissures appear on the root body, in the case of welladvanced root rot the wilted leaves lie starlike around the beet on the ground. There are possibilities of confusion with various pathogens attacking the root body. Reliable diagnosis of Late Root Rot can only be made by laboratory analysis. With good husbandry measures (including crop rotation, intercropping, avoidance of damage to the soil structure by timely use of machinery, correct nutrient application) the damage can be reduced. On severely infected areas the choice of varieties is of vital importance. Registered varieties with tolerance to Rhizoctonia solani are available. Rhizoctonia solani as pathogen of Late Root Rot has gained in importance in European beet growing areas within the last decade. With substantial farming and plant cultivation techniques the grower can assist a reduction in damage; these are an extension of crop rotation, the improvement of soil structure by timely cultivations and catch crop growing as well as the choice of varieties. Varieties have already been registered in various countries which prove that KWS has rapidly reacted to the farmers' demands. In addition, current work in the seed technology field focus on the control of Rhizoctonia solani by means of seed treatment with special fungicides and/or antagonists, in order to prevent an early infection with the disease. Meanwhile, high-performing varieties are available with a combined resistance to Rhizoctonia and Rhizomania. Thus, KWS makes a substantial contribution to an environmentally beneficial growing method in favour of a sustainable agriculture. Root Rots found in many sugar beet clamps in the UK have a number of origins. They can originate from the field from infections on roots suffering mechanical, pest or drought damage, or poor soil structure. However, in addition to good cultural husbandry in the field, attention should also be paid to constructing a free draining, well ventilated clamp.