In vitro Evaluation of the Effectiveness of Some New Fungicides in the Control of Rhizoctonia solani in Tobacco Seedlings

Healthy and high-quality tobacco seedling is the basis for successful tobacco production and for obtaining good yield and quality. But, production of such seedling must be protected to a maximum in order to prevent diseases or to achieve minimum intensity of attack and minimum damage.

The most destructive disease that attacks tobacco seedling is the damping off disease. It can occur in all stages of seedling growth, starting from seed germination to rapid growth. Sometimes it appears immediately after germination and prevents further development of the seedling.

The causing agent of the disease inmost cases (besides Pythyum debarianum) is the pathogenic fungus Rhizoctonia solani (Kühn). The symptoms of disease begin with occurrence of small watery spot at the base of the stem which begins to spread. The tissue becomes necrotic and the movement of water and nutrients is disabled. The plant falls down on the surface of the seedbed, as if it was cut. Infection spreads rapidly, infected patches in the seedbed increase and if protective measures are not taken, they occupy larger area. The damage is evident, especially in the early stages of growth. The soil pathogen is difficult to control because Rhizoctonia solani is common in various soil types and has many hosts. The fungus causes serious damage to many important field and horticultural crops (Nunez, 2005). Also, the application of chemicals is more difficult because the number of preparations for protection of tobacco seedling is limited and inappropriate fungicides are used in practice, which are not efficient in the control of the causing agent.

Several researchers studied the effectiveness of various active ingredients for control of this pathogen. According to Csinos and Stephenson (1998), the active ingredients flutolanil, iprodione, fluazinam and tebuconazole were effective in preventing the infection in tobacco seedbeds.

Fungicides that contain PCNB (Terrachlor), Iprodione (Rovral) or Azzoystrobin (Quadris) are effective against R. solani (Koenning, 2007), as well as Difenokonazol (Dahmen and Staub, 1992). According to Boogert and Luttikholt (2004), pencycuron and flutalonil are specific active ingredients for Rhizoctonia control. Azoxystrobin and trifoxystrobin are also reported as highly effective. Mocioni et al. (2003) reported that tebuconazole applied in 3 different formulations is also highly effective against R. solani. This was also confirmed by Breneman and Murphy (1991).

Singh et al. (2015), infield trials as well as in vitro, obtained the best results with carbendazym 25% + flusilazole 12.5% SE (NS) compared to the other combination (trade name) of these active ingredients, as well as kresoxime-methyl 44.3% and validamycine 3% L.

Among 14 fungicides with different active ingredients tested in vitro against R. solani, Metalaxyl, Mancozeb, Tricyclazole, Thiophanate methyl and Carbendizim + Mancozeb (all at concentration of 0.1%) were most effective in suppressing the development of the fungus (Srinivas et al., 2014 ). Sriraj et al. (2014) tested several fungicides with active ingredients azoxystrobin, tebuconazole, tridemorph, fosetyl-aluminum, trifloxystrobin, fenamidone + mancozeb, carbedazim, tebuconazole + trifloxystrobin and reportedthat inhibition percentage increased with higher concentrations, but it differed depending on the active ingredient.

Anjana and Kumar (2008) studied the efficacy of six different active ingredients-carbendazim, mancozeb, hexaconazole, propiconazole, copper oxychloryde and thiram forcontrol of R. solani under in vitro conditions in soybean. They found that all fungicides show variable inhibition of radial growth of the pathogen depending on the concentration, but propiconazole, hexaconazole and carbendazim reached the highest efficacy at all concentrations.

Rajput et al. (2016) tested 16 fungicides (contact, systemic and combinations)- and one antibiotic and found high inhibition effect in fungicides of each group and combination, including the antibiotic Validamycin.

Rajendra prasad et al. (2017) conducted extensive in vitro studies on the efficacy of various fungicides and combinations, as well as biocontrol agents-fungal and bacterial, from which useful recommendations can be obtained for both chemical and biological method of pathogen control.

Recently, many new fungicides and active ingredients for control of the pathogen have appeared on the market. The most popular against R. solani in soybean are the strobilurins pyraclostrobin and trifloxystrobin (Mueller, 2014), but Signum fungicide-combination of pyraclostrobin and boscalid-is more and more recommended. It has preventive and systemic activity for use against certain diseasesin various crops (BASF, 2008). Such a combination provides a wide range of activities and reduces the risk of resistance to various pathogens (Hauke et al., 2004). The lack of data on the use of Signum and its application in tobacco urged us to include this fungicide in our trials.

Azoxystrobin (Quadris) is a fungicide with a wide range of protection from plant diseases (Syngenta, 2006). It is also recommended in the control of target spot on tobacco caused by R. solani AG-3 (Bertrand, 2012). According to LaMondia (2012), the treatment of tobacco seedling with Quadris should be a key measure for control of R. solani. Despite the promising results for the last three active ingredients, there are scant data on the inhibitory effect of mycelial growth of R. solani.

The aim of this study was to investigate the efficacy of several fungicides for control of pathogenic fungus R. solani under in vitro conditions. The obtained results will make a great contribution to expand the list of fungicides for the control of this pathogen. Application of highly efficient fungicide against the cause of the disease will provide high protection on one hand and lower costs of the protection products, on the other.