The UK Narcissus industry continues to experience increasing levels of basal rot caused by Fusarium oxysporum f.sp. narcissi (FON). The disease is probably the most serious of those affecting Narcissus and as for other F. oxysporum f.spp., FOC produces long-lived chlamydospores which makes management difficult. Growers rely on only two fungicide active ingredients for the control; chlorothalonil (e.g. as Bravo 500) and thiabendazole (as Storite Clear Liquid or Tezate 220 SL), both of which are available for dipping bulbs including by hot-water treatment (HWT) through EAMUs. This situation leaves the industry vulnerable to a loss of disease control options as a result of legislation or commercial considerations.
To address this,work at Warwick funded by HDC (BOF 74) with Gordon Hanks has sought to identify new fungicides which can control FON in HWT. Initial tests on agar showed that products based on prochloraz (an imidazole), tebuconazole (a triazole) or copper oxychloride were effective at suppressing FON growth. Results for the currently approved fungicides chlorothalonil and thiabendazole showed that the former was relatively ineffective except at higher concentrations, while the latter failed to adequately reduce growth of two of the eight FON isolates tested, suggesting pathogen tolerance to this a.i.In addition, approval for thiabendazole will be lost at the end of 2015 and there are currently no clear plans from manufacturers to apply for re-registration of this product. However, further work in HDC BOF74a demonstrated that Orius (tebuconazole), Mirage (prochloraz) and Storite (thiabendazole) gave good control of basal rot for HWT bulbs planted in growing medium inoculated with FON with Orius (tebuconazole) being the most effective. The HDC are therefore currently pursuing an EAMU for tebuconazole for use in HWT of Narcissus bulbs.