During spring 2019, a sugar beet (Beta vulgaris cv. Barrosa) crop with dull green and chlorotic foliage was observed (Figure 1) in the commune of Dar Ould Zidouh, Fkih Ben Salah province, Morocco. The disease incidence was 21% over an area of 3.5 hectares, and there appeared to be a significant reduction in yield. Four diseased sugar beet samples were collected from two different fields. The roots showed numerous circular black-to-brown necrotic lesions, while severely infected roots were completely rotten (Figure 2).

         Infected roots were cut into small pieces which were then surface disinfected by immersion in a 2% NaOCl solution for one minute (Farhaoui et al., 2023). Pieces were placed on potato dextrose agar (PDA) and incubated at 25°C for seven days. Using single spore isolation, five isolates were purified. Colonies were initially white with a feathery surface and after 15 days incubation, they turned creamy yellow (Figure 3). All four isolates had a morphology compatible with Clonostachys rosea (Schroers et al., 1999). The fungus produced penicillate and verticillate conidiophores similar to those described by Afshari & Hemmati (2017). The hyphal diameter was 7.9 to 8.8 μm. Conidia were 6.9 μm × 9.9 μm (Figure 3). Genomic DNA from one isolate was extracted and PCR was performed using primers for the partial sequence of the internal transcribed spacer (ITS) region (ITS1/ITS4) and the beta-tubulin gene (Bt1a-Bt1b) (Glass & Donaldson, 1995). PCR products were sequenced. BLASTn analysis revealed that the ITS (GenBank Accession No. MZ854090) and β-tub sequences (OQ076272) had 98.27% and 98.92% identity with those of C. rosea (OM965347 & MT845995, and KX185040, respectively). Phylogenetic analyses based on the neighbour joining technique confirmed identification of the isolate as C. rosea (Figure 4).

        The isolate was used in a greenhouse test to confirm the pathogenicity on sugar beet cv. Barossa. Wheat kernels infected with C. rosea were used as inoculum (Afshari & Hemmati, 2017). Inoculation was done at the two-leaf phase of the plants using five colonised wheat kernels placed around the crown of each plant. In control treatments, sterile wheat kernels were used All pots were placed in the greenhouse at 25 ±2°C. Four repetitions were used for each treatment. Seven weeks later, plants were harvested and root rot evaluated. Inoculated plants showed rot roots similar to those observed in the field (Figure 5). Control plants were symptomless. The pathogen was re-isolated from diseased roots and confirmed to be C. rosea based on morphological criteria.

        Clonostachys rosea has frequently been documented as a saprotroph or mycoparasite in soil and diverse plant materials (Yuan et al., 2021). However, recent studies have shown that C. rosea is a pathogen and causes root rot in faba bean (Afshari & Hemmati, 2017) and soybean (Bienapfl et al., 2012). To our knowledge, this is the first report of the occurrence and pathogenicity of C. rosea on sugar beet in Morocco and globally.

        Source of information:https://bsppjournals.onlinelibrary.wiley.com/doi/full/10.1002/ndr2.12235