Epidemiology and control of Fusarium ear blight

Abstract

A series of related investigations was undertaken during the three year period 1993 - 1995 to investigate various aspects of the epidemiology of Fusarium ear blight on wheat in the UK, the effect of the disease on yield and control of the disease using fungicides.

The effects of ear blight caused by Fusarium culmorum on yield and yield components was investigated for three years in small plots inoculated at GS 65 and irrigated subsequently. There was a strong linear association between ear blight severity and thousand grain weight, assessed either on whole plots or on single tillers.

There were no significant differences between data obtained by the two methods, nor were differences between the three years significant. Amalgamating all data for the three years, a statistically significant yield loss relationship was determined, which indicated that y = 47.72 - 0.22x, where y = thousand grain weight and x = percentage of spikelets infected.

The possibility that ears may become infected systemically from stem-base lesions caused by F. culmorum, F. graminearum and Microdochium nivale was studied in a glasshouse under conditions designed to avoid the splash dispersal of conidia from infected compost. Each species was recovered from stem tissues above soil level in some, apparently symptomless, plants.

There was an inverse relationship between recovery of the pathogen and the height above stem base from which the stem tissue was excised. F. culmorum was the most frequently isolated fungus and it was also recovered from the highest position in plants. Only 3% of plants were colonised above the second node and none of the fungal species were recovered from either the fifth node or the ear. This suggests that systemic colonisation of winter wheat from Fusarium infected stem bases is unlikely to contribute to the development of ear blight symptoms in winter wheat.

Small-scale field experiments were carried out to determine the effect of increasing relative humidity on the development of the five major species of Fusarium known to cause ear blight in the UK. A mist irrigation system, controlled by an in-plot relative humidity sensor, was used to manipulate relative humidity levels in plots artificially inoculated with single species of either F. avenaceum, F. culmorum, F. graminearum, F. poae or Microdochium nivale.

The degree of ear infection by all five inoculated species was related to relative humidity, with evidence of differing optima. Yield data indicated that losses could occur even under conditions of low relative humidity, and that F. culmorum and F. graminearum were the most aggressive pathogens, capable of reducing yield by up to 25%.

The mycotoxin content of grain increased with increasing humidity, with highest levels of mycotoxin contamination being associated with infection by either F. culmorum or F. graminearum. Mean levels of mycotoxins were low, and not significant in terms of risk to human or animal health, but one sample from grain infected with F. culmorum did have mycotoxin levels above the guidelines on mycotoxin content used in Canada.

The relationship between Fusarium ear blight and meteorological factors was studied using data from the ADAS/CSL winter wheat disease surveys for the period 1988-1995 and from monthly weather reports from the Meteorological Office. Seasons with high levels of Fusarium ear blight were those with wetter than average conditions in June and July, following a dry November and warmer than average January.

In an evaluation of fungicides for control of Fusarium ear blight, and also black point (Alternaria alternate), few showed activity against either disease. Products containing tebuconazole gave some control of ear blight, and tebuconazole plus triadimenol plus anilazine reduced black point incidence when applied at GS 75. Irrigation in early July markedly increased the incidence of black point.

In experiments on its protectant and eradicant activity, tebuconazole was applied on a series of dates before or after inoculation at early anthesis with F. culmorum, and irrigation was provided after inoculation. Ear blight was reduced by up to 90% by tebuconazole applied up to seven days after inoculation, but there was protectant activity only when applied two days before inoculation. At non-irrigated sites, Fusarium ear blight incidence was very low in each of the years 1994-1996, and there was no effect on yield of tebuconazole applied at or after GS 57.