Integrated control of fusarium ear blight (PhD)

Fusarium ear blight (FEB) is a significant disease of small grain cereals and has been reported throughout the world. The disease is caused mainly by five mycotoxigenic species, Fusarium culmorum, F. graminearum, F. avenaceum, F. poae and Microdochium nivale. These fungi survive and sporulate on crop residues, and infect subsequent crops at the flowering stage. High temperature and humidity play an important role in infection. Initial symptoms of FEB commonly appear 4-5 days after infection. Symptoms are generally the same in all small grain cereals and are initially similar for all the species causing FEB in the UK. Small, water-soaked brownish spots develop at the base or middle of the glume or on the rachis. Infected spikelets then senesce and take the typical colour of ripe ears in contrast with the green uninfected ears.

Grain harvested from FEB-infected ears is often small and shriveled and may contain mycotoxins (e.g. deoxynivalenol) produced by the fungi. The risks connected with the consumption of contaminated food products by livestock and humans must therefore not be ignored. In June 2005 the EC advised action to be taken on trichothecenes and proposed regulatory limits, which will apply from 1 July 2006 (EC regulation No 856/2005). For deoxynivalenol (DON) the limits include 1250 ppb for unprocessed cereals, 750 ppb for cereal flour and pasta, 500 ppb for retail products such as bread, pastries, biscuits, cereal snacks and breakfast cereals, and 200 ppb for baby food.

Due to change in agricultural practice (no-tillage) and in climatic conditions, FEB and therefore, the contamination of grain with mycotoxins, are posing an increasing risk to animal and human health. The availability of control measures remains limited. Control options consist of reducing the amount of inoculum available to cause the disease. Cultural practices such as crop rotation can reduce the carry-over of spores or other survival structures between seasons. Once the crop is exposed to the pathogen, a further set of control strategies must be considered, including genetic resistance, chemical treatment and biological agents. Breeding programs have greatly aided in the eradication of very susceptible varieties. However, the resistance appears to be in most cases under polygenic control making the development of resistance cultivars with suitable agronomic and quality traits a challenge. Therefore, such efforts do not offer an immediate protection against FEB. At the present time, there is no fungicide to control FEB with a 100% efficacy. Among the active ingredient in the tested fungicides, tebuconazole has been reported the most effective for controlling FEB (Magan et al., 2002; Homdork et al., 2000; Menniti et al., 2003; Simpson et al., 2001; Cromey et al., 2001 and 2002). 

In most cases, inadequate control of FEB by chemicals is due to incorrect timing of application. Timing of fungicide application is indeed crucial for effective FEB control. Infection usually occurs during mid-anthesis, the period between growth stages 65 to 71 being the most susceptible for FEB infection (Lacey, Bateman and Mirocha, 1999). The efficacy of fungicides also depends on the timing of infection. Matthies and Buchenauer (2000) found that fungicide applications early post-infection, 2 days after inoculation, provided the most effective control against FEB while pre-inoculation and late postinoculation applications (9 days after inoculation) were less effective.

Micro-encapsulated fungicides could prove effective against FEB. They could indeed increase the length of activity of the active ingredient but also reduce cost as smaller quantities of fungicides would be needed and there would be no need for multiple applications. Over 100 micro-encapsulation processes have been described, amongst them, the micro-encapsulation of active ingredient in yeast cells (Pannell, N.A., European patent 242135). The technology uses strains of the yeast Saccharomyces cerevisiae commercially available in the baking and brewing industries. The viability of the yeast cells is not required but intact cell membranes and cell walls are essential for efficient encapsulation. The process uses only water, yeast and the active ingredient to be encapsulated. The yeast cells are dispersed in water with top stirring. The active ingredient is then added to the dispersed yeast cells and the suspension mixed until the majority of the active ingredient has entered the cells. Encapsulation levels generally attain 30 to 40 % (w/w) but can sometimes reach 80 % (Nelson and Crothers, 2003). Yeast cells containing the active ingredient are then washed with water or another appropriate solvent and spray dried. The technology has been used successfully in the food industry. Encapsulated essential oils and synthetic flavours have successfully been encapsulated and are released from the capsule on contact with the moist tongue surface without the yeast cell being disrupted. 

A UK-based company, Micap plc. developed and provided a microencapsulated formulation, containing the active ingredient tebuconazole, using the technology described above. The formulation is available as a powder that can be used readily for seed treatments or mixed in water for foliar treatments. The yeast cells are 5 to 10 μm in diameter, which make them small enough so that the formulation do not clog the equipment used during the spraying process.

The project aimed at testing this novel tebuconazole-encapsulated fungicide provided by Micap plc. The efficacy of the microencapsulated fungicide to control FEB was evaluated in planta and compared to that of various foliar fungicides. The study also aimed at isolating and screening biological control agents in the view of an integrated approach to control FEB. Chemical and biological treatments were tested in controlled environment before being assessed for their efficacy in the field.