SETTING A RATING SCALE FOR ASSESS Rhizoctonia solani LESIONS ON COTTON , SOYBEAN AND COMMON BEAN SEEDLINGS

Diseases incited by soilborne fungi are responsible for reducing the yield and cause significant impact to almost all crops. Among then, Rhizoctonia solani AG-4 is considered the most important in the cotton, common bean and soybean crops in Brazil. The use of diagrammatic scale or rating scales, as a tool to help in the quantification of the severity of a particular disease, is more common for foliar diseases. Considering the lack of a standardized, illustrated and easy-to-apply methodology for assessing the severity of R. solani lesions in cotton, soybean and common bean seedlings, a simple and precise rating scale was developed with the objective to fill this gap. The proposed scale shows four levels of disease severity, with the descriptions and illustrations for each type of lesion observed in the cotton, soybean and common bean seedlings. This developed scale was validated in many experiments and proved to be adequate for severity assessments of R. solani lesions on cotton, soybean and common bean seedlings.

Diseases incited by soilborne fungi are responsible for reducing the yield and cause significant impact to almost all crops.However, those that occur in the early stage of seedling development are considered a major problem worldwide (GOLL et al., 2013;ZEUN et al., 2013).When the conditions in the field are favorable to its occurrence, the losses in the stand and, consequently, in the grain yield, can be significant (RIZVI e YANG, 1996).In general, the damages at this crop stage are related to the reduction of seedling population.Damping-off occurs when germinating seedlings are infected prior to or just after emergence.This pathogen causes both, preemergence damping-off (rotting the seeds before and soon after germination) and post-emergence damping-off (causing the death of the seedlings newly emerged) as well as foliar blight and also stem and root rot (DORRANCE et al., 2003;GOULART, 2007aGOULART, , 2007bGOULART, , 2010;;MALVICK, 2017;RIZVI;YANG, 1996).
Based on importance, pathogenicity, occurrence and damages criteria, the soilborne pathogen Rhizoctonia solani Kuhn anastomosis group (AG)-4 (teleomorf: Thanatephorus cucumeris (A.B.Frank) Donk) is considered the most important in the cotton, common bean and soybean crops in Brazil (GOULART, 2005;2010).This importance is related to its frequency of occurrence, predominance in the majority of the soils and in different environmental conditions and, consequently, the damages that causes in the early stage of crop establishment (BIANCHINI et al., 1997;GOULART, 2007bGOULART, , 2016;;SARTORATO et al., 1987).
Pathogen such R. solani has a wide host range; it is non-specialized and polyphagous pathogen or generalist.Damage caused by this pathogen may occur at any time during the growing season, but it is more severe on young seedlings (GOULART, 2010;2016).Symptoms vary according to the host and the plant part affected.Seedling damping-off is probably the most common disease caused by R. solani.This pathogen also can cause seed rot, root rot, and lesions on hypocotyls.On hypocotyls, lesions are reddish-brown and sunken.Diseased seedlings collapse from the firm, dry canker that girdles the hypocotyl (GOULART, 2017b;2016;ZAMBOLIM et al., 1997;HARPER, 2012).Damping-off usually occurs in a period from pre-emergence and 10 to 15 days after seedling emergence (GOULART, 2010;2016).In adult plants, the symptoms of R. solani are dry root rot, sunken cankers, reddish, circular to elongated and brown lesions at the base of the stem, below and at ground level.The disease may progress, causing damping-off, wilting or temporary survival of the plants with presence of adventitious roots above of the affected region (GOULART, 2007b;HARPER, 2012).
Despite the improved control of this disease, losses are significant.Results obtained in the USA by Tachibana (1968), cited by Dorrance et al. (2003), showed that R. solani caused a loss of 50% in soybean grain yield.In Canada, studies have shown that R. solani caused a loss of 48% in stand establishment and 52% in seed yield in soybean (CHANG et al., 2018) According to McLean and Gazaway (2000), in the USA, in 1995, the reduction of cotton yield due to early-season diseases has been estimated in 180,000 tons, representing loss of 2.8% per year.In common bean, yield losses ranged from 10% to 60%, when R. solani acts as a complex with other soil pathogens (ZAMBOLIM et al., 1997).This survey has not been done yet in Brazil, considering soybean, cotton and common bean crops.
In plant disease experiments, individuals of each experimental unit are commonly classified, such as leaves, branches, fruits, flowers, according to categories of a discrete quantitative scale, corresponding to ranges of disease severity (CZERMAINSKI, 1999).In general, this classification occurs through visual assessment and results in the counting of elements or incidence by class.
The simple observation of the occurrence of a particular disease, its incidence and even severity quantification using subjective criteria (visual assessment) in most cases may lead to error, requiring the adoption of objective criteria for quantifying (AMORIM, 1995;LENZ et al., 2009).Several strategies have been proposed to quantify the severity of disease accurately.Among them, there are the diagrammatic scale or rating scales, which are illustrated representations of parts of the plant to be assessed, with symptoms of different severity levels (AMORIM, 1995;BERGAMIN FILHO e AMORIM, 1996).These scales should present a standard symptomatology, comparable to those observed under natural conditions of infection or even produced through inoculation.Thus, the use of correctly elaborated diagrammatic scales can reduce the subjectivity of severity estimates, improving the accuracy and precision of the evaluation, being the main severity assessment tool for many diseases (AQUINO et al., 2008;LENZ et al., 2009;MARTINS et al., 2004).However, it should be taken into account that the diagrammatic scales should be easy to use, applicable in different conditions, as well as provide sufficient information that represent all stages of disease development (BERGAMIN FILHO e AMORIM, 1996).
The use of diagrammatic scale or rating scales, as a tool to help in the quantification of the severity of a particular disease, is more common for foliar diseases.In relation to soilborne fungi, and specifically regarding to R. solani, two references were found about the subject, in the patosystem common bean x R. solani root rot (RODRIGUES et al., 1999; VAN SCHOONHOVEN and PASTOR-CORRALES, 1987).It should be noted that in only one of these publications, illustrations of parts of plants with symptoms at different severity levels were available, as suggested by Amorim (1995) and Bergamin Filho and Amorim (1996).For soybean and cotton crops, no reference about this subject has been found.
In experiments of fungicides seed treatment and germplasm evaluating reaction to Rhizoctonia solani soilborne, in the seedling stage, the most common is the determination of seedling dampingoff.In the remaining seedlings, only the presence of pathogen lesions in the roots and in the hypocotyl is measured (disease incidence = percentage of infected seedlings in a sample or population).The disease severity, which is estimated on the basis of tissue area covered by the symptoms, is not often considered, due to the lack of adequate and standardized visual methodology to estimate it in a safe way.Providing real images of different types of R. solani lesions on cotton, soybean and common bean seedlings, instead of drawings, for example, increases the accuracy of the evaluation.Photos are of practical use, unlike textual descriptions that may have different degrees of subjectivity and interpretation.Many experiments were carried out at Embrapa Western Agriculture, Dourados, MS, Brazil, in order to validate this rating scale, which proved to be adequate for severity assessments of R. solani lesions on cotton, soybean and common bean seedlings.In addition, the results obtained were also used to calculate the disease index (severity), in the remaining seedlings, according to McKinney formula (McKinney, 1923)).
Considering the lack of a standardized, illustrated and easy-to-apply methodology for assessing the severity of R. solani lesions in cotton, soybean and common bean seedlings, a simple and precise rating scale was developed with the objective to fill this gap.

Rating Scale
A rating scale with four levels of disease severity (0 a 3), according to type of lesions observed in the cotton, soybean and common bean seedlings, was developed and validated.Figures 1, 2 and 3 show the descriptions and illustrations for each type of lesion, its respective score and the corresponding crop.

Light intensity lesion
Only one lesion in the hypocotyl, necrotic, reddishbrown colour, which may be circular or elongated, small (less than or equal to 0.5cm), superficial and without constriction.

Moderate intensity lesion
Elongated and necrotic lesion, reddish-brown colour, in the root (A) or in the hypocotyl (B), measuring between 0.5cm and 2.0cm, superficial and without constriction.

Severe intensity lesion
Elongated, necrotic and extensive lesion (greater than 2.0cm), reddishbrown colour, covering the hypocotyl and the root system, with constriction.

Light intensity lesion
Only one necrotic lesion in the hypocotyl, predominantly in the seedling stem region, reddishbrown colour, elongated and sunken, small (less than or equal to 0.5cm)

Moderate intensity lesion
Elongated, necrotic and sunken lesions, which may be one or more, isolated or coalesced, reddish brown colour, present in the hypocotyl and/or root system, measuring between 0.5cm and 2cm Setting a rating… GOULART, A. C. P.

Severe intensity lesion
Elongated and extensive lesion (greater than 2.0cm), reddishbrown colour, necrotic, covering the hypocotyl and the root system, with constriction

Score
Lesion Description Symptoms 0 Healthy seedlings (no visible symptom) 1

Light intensity lesion
Only one necrotic lesion in the hypocotyl, predominantly in the seedling stem region, reddish-brown colour, elongated and sunken, small (less than or equal to 0.5cm) 2

Figure 1 :
Figure 1: Rating scale for assessment of severity of Rhizoctonia solani lesions on cotton seedlings

Figure 2 :
Figure 2: Rating scale for assessment of severity of Rhizoctonia solani lesions on soybean seedlings.

Figure 3 :
Figure 3: Rating scale for assessment of severity of Rhizoctonia solani lesions on common bean seedlings.