SYSTEMIC GROWTH OF SEVERAL FUSARIUM CROWN ROT CAUSAL AGENTS AND DEOXYNIVALENOL TRANSLOCATION FROM THE STEM BASE TO THE HEAD OF SOFT WHEAT

Fusarium graminearum, F. pseudograminearum and F. culmorum are considered the principal causal agents of Fusarium Crown Rot (FCR) of wheat. These three species are also able to biosynthesize the mycotoxin deoxynivalenol (DON) during plant colonization, representing a possible additional source of grain DON contamination. To further investigate these aspects, in the present study, soft wheat seedlings of the cultivar Genio were inoculated at the stem base with conidial V8-agar suspensions of the DON-producing strains Fg3005 (F. graminearum), Fp3427 and Fp3096 (F. pseudograminearum), Fu5 and F820 (F. culmorum). At maturity, symptoms were evaluated and plants sectioned into four segments: segment 1 (inoculated area), segment 2 (central part of the plant), segment 3 (last internode) and segment 4 (head). Fungal colonization was assessed by quantitative real-time PCR (qRT-PCR) using specific primers for each of the inoculated species, while, TEF1α primers were used for plant DNA quantification. Histopathological analyses were conducted by Scanning Electron Microscopy (SEM). The mycotoxins DON, 3-acetyl-DON (3AcDON), 15-acetyl-DON (15AcDON) were determined by high performance liquid chromatography coupled with tandem mass spectrometry.(HPLC-MS/MS). Strain Fp3096 showed the highest virulence in terms of FCR browning symptoms, followed by F820, Fg3005, Fu5 and Fp3427. Disease symptoms were observed up to the third node (segment 2) in the plants inoculated with Fp3096 and F820 while in the other cases symptoms, with a different incidence, were limited to the crown-second node area (segment 1). No symptoms were detected in the last internode (segment 3) or in the head (segment 4). All inoculated strains, even if at different amounts, were detected by qRT-PCR up to the third node (segment 2). Only Fp3096 and F820 strains were present in the last internode area. None of the inoculated strains was detected in the head tissues. Mycotoxin analyses showed the presence of DON and 3AcDON. With the exception of segment 4 (head) of the plants inoculated with Fp3427 (the least virulent strain), all other segments were contaminated by DON+3AcDON. Plants inoculated with F820 and Fp3096 showed the highest DON+3AcDON contamination on segments 1, 2 and 3. The heads of plants inoculated with F820, Fp3096, Fg3005 and Fu5 were found to be contaminated with DON+3AcDON, even in the absence of the fungus. These preliminary results show that three typical FCR causal agents systemically grew beyond the third node following stem base inoculation and that in some cases a limited presence of the most virulent strains was detected in the last internode. However, even if none of the fungi were able to colonize the head, DON+3AcDON synthesized by the pathogens were detected in the heads. In particular, it is interesting to observe that F. pseudograminearum, a typical wheat crown pathogen, contributed to head contamination by DON.