Molecular characterization of Rhizoctonia spp. isolates and sustainable approaches to control Rhizoctonia diseases in ornamental nursery

Eighty-eight Rhizoctonia isolates were recovered from 30 ornamental species grown in nurseries in Sicily and their anastomosis group (AG) and subgroups were determined. Fifty-six isolates belong to binucleate Rhizoctonia BNR and 32 were multinucleate and identified as R. solani. The binucleate AG most frequently found in this survey was AG-R which has not been reported until now in Italy. Overall, disease data surveys correspond to the first reports of diseases caused by binucleate Rhizoctonia AG-A, AG-G, and R. solani AG-4 on Citrus volkameriana, Citrus sinensis × Poncirus trifoliata (Citrange), Trachycarpus fortunei, Arbutus unedo, Thevetia peruviana Bignonia sp., Carissa spp., Catharanthus roseus, Eugenia sp., Quercus ilex, Phyllirea angustifolia and Pittosporum tobira. The high frequency and disease incidence detected through surveys in ornamental nurseries located in Sicily clearly show as this fungus represents a serious threat for ornamental plant production. Based on these considerations, the research topics of this PhD thesis was focused to develop an effective integrated control program in controlling both Rhizoctonia infections in nursery and in suppressing and/or strongly reducing of resting structures (sclerotia) into soil. Therefore, following an in vitro screening of some BCAs effective against R. solani (AG-4 group) and Rhizoctonia BNR (AG-A, AG-G and AG-F groups), the consequent step was the efficacy evaluation of 6 commercial fungicides and 8 microbiological formulates under controlled environment (microcosms) for the control of Rhizoctonia crown and root rot. To this aim, a plant host-pathogen model was successfully set up by using Dodonea viscosa. Overall, these data clearly indicate that only tolclofos-methyl can be employed for chemical management of Rhizoctonia infections, although encouraging results were found for other fungicides. Regarding the valid measures to suppress pathogen inoculum, the efficacy of some fumigants was tested at label and sub label rates in 5 nursery experiments under different applicative conditions. Interesting data in suppressing Rhizoctonia survival were obtained even though in plots fumigated with low rates (up to 5-fold reduced) of dazomet and metham sodium, whereas viable Trichoderma colonies were almost always retrieved. However, TIF barrier has averagely enhanced retention fumigants capacity if compared to VIF, thus significantly increasing their efficacy in suppressing fungal inocula and viability of Trichoderma spp. Further experiments, performed to evaluate efficacy of a new fumigant (DMDS) in controlling Rhizoctonia inocula in cultivation substrates, clearly show as it could be supported as a valid alternative to standard fumigants, although it was actually registered against Meloydogine spp., Heterodera spp., Globodera spp and Pratylenchus spp. nematodes. Comprehensively, the present PhD thesis provides useful information about diffusion and control of Rhizoctonia disease on ornamental nurseries.