First report of a new binucleate Rhizoctonia on potato tubers in the UK

A sample of potato tubers of cv. Navan displaying growth cracking (up to 10 mm in depth and width) and elephant hide symptoms often associated with Rhizoctonia potato disease were received by Fera for diagnosis. The tubers, originating from a commercial farm in Cornwall UK, were harvested in October 2009. For each symptom, pure Rhizoctonia cultures were obtained as previously described (Woodhall et al., 2007) and DNA was extracted from the resulting cultures and tested for R. solani anastomosis groups (AGs) 2-1, 3, 5, and 8 using real-time PCR (Budge et al., 2009). AG-3 was consistently found from tuber areas showing severe elephant hide, sunken areas and growth cracking symptoms. However, isolates consistently recovered from areas of less severe elephant hide symptoms, which often covered as much as 50% of the tuber surface (Fig. 1), could not be assigned to a specific AG by this method. 

To determine the identity of these isolates, the hyphae of one representative isolate was stained with trypan blue in lactoglycerol (0.1% stain made with 67ml lactoglycerol, 33ml distilled water and 0.1g trypan blue) to visualise nuclei. Under 400x magnification, each cell had two nuclei indicating the strain was a binucleate Rhizoctonia species (BNR). The mean width of 20 mature hyphae was 5.1 µm (range 3.81 to 7.63 µm, s.d.= 0.89 µm) and consistent with BNR species. Since previous studies have demonstrated the utility of DNA-based assays to identify AG in the absence of tester isolates for typing, sequencing of the rDNA ITS region was undertaken for the isolate with primers ITS1 and ITS4 (White et al., 1990). The resulting sequence (GenBank Accession No. FR828480) was compared to other sequences in GenBank. No near identical matches were found suggesting the isolate could be a unique strain of a particular BNR or belong to a previously undescribed AG. The isolate was therefore deposited into the Fera culture collection as Accession No. cc43. 

To determine whether the isolate was the causal agent of the observed symptoms the following experimental protocol was carried out. Potato minitubers (cv. Santé) were placed at 100 mm depth in 2 litre pots with John Innes Number 3 compost. Three 10 mm PDA plugs from a 14-day-old culture of the isolate were placed on top of each minituber. Pots were held in a controlled environment room at 18°C with 50% relative humidity and watered as required. After four weeks, plants were removed and assessed for disease. Small lesions (≤5 mm) were present on roots and stems (Fig. 2). Infected lenticels and similar mild elephant hide symptom were present on the seed tuber. BNR was consistently isolated from symptomatic tissue.

Here, we report the finding of a previously undescribed strain of BNR occurring in a UK potato crop. This is the first account of an infection of potato caused by Rhizoctonia other than R. solani in the UK.R. solani has been associated with Rhizoctonia potato disease in UK potatoes, with a number of different isolates assigned to AGs 2-1, 3 and 5 (Chand & Logan, 1983; Woodhall et al., 2007). We further demonstrate the ability of this isolate to cause disease on potato roots and stems. Future work on Rhizoctonia potato disease in the UK should also consider examining the relative contribution of BNR to the epidemiology of Rhizoctonia diseases of potato.