Sclerotinia sclerotiorum (cottony soft rot)

S. sclerotiorum can attack young plants, causing damping-off on sunflowers (Huang and Kozub, 1990), rape (Hims, 1979), tobacco (Ivancheva-Gabrovska et al., 1978) and beans (El-Helaly et al., 1970). More frequently, S. sclerotiorum infects root tissues at later stages of plant growth, causing root rot, basal stem canker, wilt and premature ripening in many hosts, including sunflowers (Dorrell and Huang, 1978; Hoes and Huang, 1985), Jerusalem artichokes (Helianthus tuberosus) (Cassells and Walsh, 1995) and lettuces (Adam and Tate, 1975, 1976). It can also attack above-ground tissues, causing leaf blight, stem blight, blossom rot, head rot and pod rot in many plant species, including sunflowers (Huang, 1983a), safflowers (Muendel et al., 1985), beans (Abawi et al., 1975a), peas (Huang and Kokko, 1992) and rape (Gugel and Morrall, 1986). Damping-off and wilt are caused by myceliogenic germination of sclerotia (Adam and Tate, 1976; Huang and Kozub, 1990). In contrast, diseases of above-ground tissues, such as stem rot, leaf blight, head rot, pod rot and blossom rot, result from infection by airborne ascospores (Huang and Kokko, 1992). Secondary spread of the disease occurs by direct contact with infected tissues during the growing season (Murray et al., 1978; Huang and Hoes, 1980).For most of the hosts, infected roots, stems, leaves and pods first develop water-soaked lesions. The lesions expand and later become brown or bleached. Under conditions of dense canopy and wet weather, fluffy white mycelia are evident on infected tissues. The dense mycelial mats turn from greyish-white into black sclerotia which form on the surface of infected tissues or are embedded within them (Huang, 1977). Plants with root infection often wilt and die prematurely. The affected stems tend to thread and lodge under strong winds. Individual plants, and occasionally an entire crop, can be destroyed by the disease, resulting in the appearance of flat, brown patches in the field later in the growing season.

IPM Programmes

An integrated control approach using rotation of 2 years or longer, delayed seeding, increased potassium fertilizer and fungicide spray was effective in reducing sclerotinia incidence and increasing yield in sunflowers in China (Hua et al., 1994).

Cultural and Sanitary Methods

Plant spacing and row direction
For sclerotinia wilt of sunflower originating from root infection (Huang and Dueck, 1980), modifying plant spacing can be effective in reducing primary infection loci (Huang and Hoes, 1980) as well as minimizing the secondary spread of the disease by root contact (Hoes and Huang, 1985). Incidence of sclerotinia wilt of sunflowers was significantly reduced at a spacing of 25 cm or wider between plants (Hoes and Huang, 1985).

Altered plant spacing is also effective in reducing the disease caused by infection of airborne ascospores of Sclerotinia spp. For example, white mould of beans is less severe in wider rows (60-80 cm) than narrow rows (25-30 cm) (Steadman et al., 1973). Changing row direction to reduce disease caused by airborne inoculum was reported for S. sclerotiorum on beans (Haas and Bolwyn, 1973) and S. trifoliorum on forage legumes (Bennett and Elliott, 1972).

Irrigation
High soil moisture is conducive to carpogenic germination of sclerotia of Sclerotinia species. Some studies show that heavy irrigation contributes to increased severity of white mould of beans (Weiss et al., 1980) and sclerotinia stem rot of soyabeans (Grau and Radke, 1984). Carpogenic germination of sclerotia occurred more rapidly in the continuously irrigated soil (Kopmans, 1993). For irrigated crops such as lettuce and dry beans, reducing the number of irrigations during the period with high risk of disease can reduce disease in the absence of rainfall (Steadman, 1979).

Physical barriers and soil solanization
In some regions, soil mulching with transparent polyethylene film can raise soil temperatures, thus adversely affecting the survival of sclerotia. Several reports indicate that solanization was effective in reducing the inoculum of S. sclerotiorum (Porter and Merriman, 1985) and S. minor (Porter and Merriman, 1985). The polyethylene mulching inhibited sclerotial germination, disturbed apothecial formation and trapped ascospores (Ginoux and Blancard, 1984). Honda and Yunoki (1977) demonstrated the use of UV-absorbing film to prevent apothecial production from sclerotia of S. sclerotiorum and to control sclerotinia blossom rot and fruit rot of aubergines and cucumbers in the greenhouse.

Soil amendment
Organic or inorganic matter can be used in the management of soilborne diseases (Sun and Huang, 1985). Huang and Sun (1991) reported that amending soil with a formulated product, S-H mixture (Sun and Huang, 1985) was effective in suppressing apothecial production from sclerotia of S. sclerotiorum. Sharma et al. (1983) found that soil mulches with pine needles or sunflower inflorescence residues reduced incidence of sclerotinia stalk rot and increased yield in cauliflowers. Lumsden et al. (1983) reported a significant reduction in incidence of lettuce drop caused by S. minor, by the incorporation of 10% (w/w) of composted sewage sludge to the greenhouse soil. Asirifi et al. (1994) found that soil amended with fowl manure or alfalfa hay effectively reduced incidence of lettuce drop caused by S. sclerotiorum and increased marketable heads.

Crop rotation
Because of the persistent nature of the Sclerotinia pathogens, crop rotation alone is insufficient to manage the disease in many crops. Morrall and Dueck (1982) found that a 4-year rotation was ineffective for the control of sclerotinia stem rot of rapeseed. Nevertheless, crop rotation is generally considered as a good practice to prevent the build-up of sclerotia in the soil. Kirchner and Pluschkell (1973) found that the severe incidence of sclerotinia stem rot of rape resulted from short and frequent rotations of rape and cabbage. Steadman et al. (1972) suggested that white mould of beans can be managed by a combination of a 3-year rotation, low sowing rates, wide row spacing, reduction of irrigation and removal of crop residues.

Host-Plant Resistance

Genetic resistance (partial physiological resistance)
Differences in susceptibility to S. sclerotiorum, S. minor, or S. trifoliorum were reported in cultivars, breeding lines, and plant introductions of soyabeans (Grau and Bissonnette, 1974; Jiao et al., 1994), beans (Adam et al., 1973), groundnuts (Porter et al., 1975), sunflowers (Orellana, 1975), safflowers (Muendel et al., 1987), rape (Sun et al., 1982), grams (Gurdip-Singh and Gill, 1979), lettuce (Adbul-Madjid et al., 1983) and other crops.

In beans, resistance to white mould was found in some accessions, including Rico 23 and Ex Rico 23 (Tu, 1985; Middleton et al., 1995). Adam et al. (1973) reported that Phaseolus coccineus (scarlet runner bean) is resistant to S. sclerotiorum. From the crossing of Phaseolus vulgaris (common bean) and P. coccineus, B-3749, Abawi et al. (1978) found that the resistance is controlled by a single dominant gene. Other studies showed that resistance to white mould in beans was quantitatively inherited (Coyne et al., 1977) and due primarily to additive gene action (Fuller et al., 1984a).

Sunflower inbred lines CM 392 (Huang, 1980b), CM 526 (Huang, 1980b), CM 361 (Huang, 1980b; Dedio et al., 1983), HA 60 (Rogozheva et al., 1984), and HA 61-1 (Huang 1980b; Noyes and Hancock, 1981) were resistant to sclerotinia wilt caused by S. sclerotiorum. Huang (1980b) found that sunflower hybrids from the cross of resistant inbreds CM 526 x HA 61-1 was also resistant to sclerotinia wilt in the disease nursery. Significant correlation in susceptibility to S. sclerotiorum between parental lines and the derived hybrids was also reported in sunflowers (Grezes-Besset et al., 1994) and Brassica napus (Liu et al., 1991). Resistance to S. sclerotiorum in sunflowers (Dedio, 1992), cauliflowers (Baswana et al., 1991) and lettuces (Madjid, 1981) was polygenically controlled, although resistance controlled by a single recessive gene was also reported for sunflowers (Pirvu et al., 1985).

Culture filtrates of S. sclerotiorum are toxic to host plants (Huang and Dorrell, 1978). Maxwell (1973) found that extracts of hyphae of S. sclerotiorum contained an enzyme which catalyzed the formation of oxalate and acetate from oxaloacetate. The oxalic acid from the pathogen was found to be the main toxic component (Godoy et al., 1990) for inducing wilt symptoms (Huang and Dorrell, 1978; Noyes and Hancock, 1981) and macerating tissues and preparing the substrate for the activity of pectolytic enzymes (Antonova and Bekhter, 1983). Disease tolerance associated with tolerance to oxalic acid was found in bean cultivar Ex Rico-23 (Tu, 1985) and sunflower line HA61 (Noyes and Hancock, 1981). Mullins et al. (1995) reported that the high level of resistance in Brassica napus line HH1 was related to its high sensitivity to oxalic acid, resulting in the rapid production of oxidized phenols around the infection site. Oxalic acid was also used in selection of alfalfa for resistance to S. sclerotiorum and S. trifoliorum (Rowe, 1993).

Disease avoidance
Escape from infection by Sclerotinia species because of specific types of growth habit was found in many crops, including lettuce (Newton and Sequeira, 1972b), sunflowers (Malinina, 1981) and beans (Steadman, 1979; Saindon et al., 1995). Dry beans with an upright or open-bush growth habit are less susceptible to infection by ascospores of S. sclerotiorum than the dense bush or vine-like plants, because the former can create an environment that is not conducive to the production of apothecia and infection of ascospores (Coyne et al., 1974, Casciano and Schwartz, 1985). Deshpande et al. (1995) found that the high incidence of white mould in the cultivar Tara, which has a dense canopy, was associated with its long leaf wetness period. Lettuce drop due to S. sclerotiorum was reduced in cultivars with a raised growth habit (Newton and Sequeira, 1972b). Sunflower plants that were 120-150 cm tall, that have flat inflorescences with correct angle of attachment to the stem and few but erect leaves, exhibit increased resistance to sclerotinia head rot in the field (Malinina, 1981).

Senescent flower tissues play a significant role in initiating infection by ascospores of S. sclerotiorum (Hunter et al., 1978; Huang and Kokko, 1992). Fu (1990) found that the level of resistance to S. sclerotiorum in a petal-less line of rape was 85-95% higher than the petalled varieties.

In some cases, disease avoidance was linked to plant maturity. The low incidence of sclerotinia stem rot in the winter rape cultivar Lindora was due to late flowering (Ahlers, 1986). On the other hand, lines of dry bean with determinate growth habit and early maturity resulted in greater disease avoidance than lines with indeterminate growth habit or late maturity (Steadman et al., 1973; Fuller et al., 1984b). Disease avoidance related to early ripening was also found in some soyabean cultivars (Vidic et al., 1983) and the sunflower cultivar Skoropelyi (Pimakhin et al., 1984).

Biological Control

Species of Sclerotinia are difficult to control because they produce sclerotia which are well adapted to survive under adverse environmental conditions. However, numerous reports indicate that some of the mycoparasites may have potential for use as biocontrol agents for sclerotinia diseases. Among these mycoparasites, the basic biology and ecology of Sporidesmium sclerotivorum [Teratosperma sclerotivora] and Coniothyrium minitans have been the most thoroughly studied (Ayers and Adams, 1981; Whipps and Budge, 1993).

In the USA, Adams and Ayers (1982) studied lettuce drop caused by S. minor and found that a single treatment with Sporidesmium sclerotivorum [Teratosperma sclerotivora], at a rate of 100 or 1000 macroconidia per gram of soil, was effective in reducing the disease incidence by 40-83% in four consecutive crops. Another study showed that one application of S. sclerotivorum significantly reduced the incidence of lettuce drop for the first three crops; disease levels in the fourth and fifth crops were similar to those in the control plots after an increase in indigenous populations of the hyperparasite in the untreated plots (Adams and Fravel, 1990). Biological control of lettuce drop by S. sclerotivorum may be economically feasible because of the lasting effect of the control, the high cash value of the crop and the relatively low cost of treatment.

Coniothyrium minitans was effective for control of sclerotinia wilt of sunflowers in the field (Huang, 1980a; McLaren et al., 1994), and against white rot of soyabean (Sesan and Csep, 1992) and lettuce drop caused by S. sclerotiorum in the greenhouse (Budge and Whipps, 1991). Huang (1980a) conducted a 3-year study in a field naturally infested with S. sclerotiorum and found that application of C. minitans to the seed furrow reduced the incidence of sclerotinia wilt of sunflower by 42 to 56% compared with untreated plots. This finding was further confirmed in other studies in Canada (McLaren et al., 1994) and Russia (Bogdanova et al., 1986). Studies in the UK revealed that incorporating C. minitans into the soil before planting reduced the incidence of lettuce drop and increased the marketable yield of lettuce under greenhouse conditions (Whipps and Budge, 1992). Studies in the Netherlands indicated that treatment with C. minitans effectively controlled white mould of caraway caused by S. sclerotiorum (Verdam et al., 1993; Evenhuis et al., 1995).

In Canada, Huang and Kozub (1991b) observed that C. minitans was associated with the decline of sclerotinia wilt of sunflowers and thus offered long-term benefits in suppressing the sclerotinia population. The wilt decline phenomenon was persistent, and even adding sclerotia to the soil at the seeding stage did not significantly increase disease (Huang and Kozub, 1991b; McLaren et al., 1994). In Germany, Pfeffer and Luth (1990) reported that the decline of crown and stem rot of red clover caused by S. trifoliorum was associated with an increased population of C. minitans.

The success of biological control of sclerotinia lettuce drop using Sporidesmium sclerotivorum [Teratosperma sclerotivora] (Adams and Ayers, 1982; Adams and Fravel, 1990) and sclerotinia wilt of sunflower using Coniothyrium minitans (Huang 1980a; McLaren et al., 1994) has depended on a good basic understanding of the ecological relationships between the hyperparasites and their hosts (Adams et al., 1984; Whipps and Gerlagh, 1992). In their natural habitat both mycoparasites can destroy sclerotia (Hoes and Huang, 1975; Adams and Ayers, 1981). Ayers and Adams (1979a) observed that S. sclerotivorum grew well on living sclerotia but grew slowly on heat-killed sclerotia and did not grow at all on many common mycological media. Macroconidia of S. sclerotivorum germinated and infected sclerotia of S. minor in the soil (Ayers and Adams, 1979b; Adam, 1989), resulting in the production of about 15 000 new macroconidia on each infected sclerotium (Adams et al., 1984). More importantly, mycelia of S. sclerotivorum grew through the soil and infected other healthy sclerotia of S. minor (Ayers and Adams, 1985; Adam, 1987). In addition to the reduction of initial inoculum density of S. minor, S. sclerotivorum was able to parasitize new sclerotia produced on diseased lettuce in the field (Adams and Ayers, 1982; Adams and Fravel, 1990).

Although C. minitans attacks both growing hyphae (Huang and Hoes, 1976; Huang and Kokko, 1988) and sclerotia of S. sclerotiorum (Campbell, 1947; Ghaffar, 1972; Huang and Kokko, 1987), the control of sclerotinia wilt of sunflowers is achieved only through the reduction of sclerotia, the primary inoculum (Huang, 1980a). The mycoparasite is ineffective in preventing secondary spread of the disease by root-to-root contact (Huang, 1980a).

There are several reports on successful biological control of diseases caused by airborne ascospores of S. sclerotiorum. Zhou and Reeleder (1989) found that spraying spores of Epicoccum purpurascens [Epicoccum nigrum] on snap beans at the flowering stage significantly reduced white mould under both greenhouse and field conditions. Xue et al. (1991) reported a 20-25% reduction in sclerotinia disease of oilseed rape by spraying the plants with Bacillus cereus. In a greenhouse study, Huang et al. (1993b) reported that spraying pea plants with an antagonistic strain of Bacillus cereus was effective in reducing incidence of pod rot caused by infection with airborne ascospores of S. sclerotiorum. Control of ascospore infection may be feasible by application of biocontrol agents to senescent tissues such as flower petals (Zhou and Reeleder, 1991; Hutchins and Archer, 1994).

Some reports indicate that seed treatment with a biocontrol agent (Illipronti and Machado, 1993) or a combination of a biocontrol agent and a fungicide (Zazzerini and Tosi, 1985b) is effective in controlling sclerotinia diseases. Treatment of cucumber seeds with Trichoderma viride reduced sclerotinia disease and increased yield (Fedorinchik and Buga, 1972). Incidence of sclerotinia wilt of sunflowers is reduced by seed treatment with Bacillus subtilis (Zazzerini and Tosi, 1985a), Pseudomonas fluorescens, P. putida (Expert and Digat, 1995) or a combination of Trichoderma viride and iprodione (Zazzerini and Tosi, 1985b).

Figueiredo et al. (2010) found that eight isolates of Trichoderma spp. showed antagonistic potential against S. sclerotiorum infecting bean in vitro. In vivo, application of isolate 3601 gave a 37.04% reduction in pathogenicity.

Microscopic studies showed that Pseudomonas chlororaphis strain PA-23 inhibited the germination of S. sclerotiorum ascospores on petals in greenhouse and field conditions, compared to the complete colonization of petals which was observed 48 h after application of ascospores alone. Field studies over a period of two years indicated that disease control with PA-23 and Bacillus amyloliquefaciens (BS6) was comparable to that achieved with the fungicide Rovral Flos (iprodione). There was no significant difference between single- and double-spray application of PA-23 and BS6 in the management of canola stem rot. Results suggest that P. cholororaphis PA-23 and B. amyloliquefaciens BS6 can be used to control Sclerotinia stem rot of canola under field conditions (Fernando et al., 2007).