Crop Clinic

Fungicide protection can be severely tested during rapid growth of the haulm because of the considerable new growth produced between fungicide applications.

Protecting new growth is obviously more difficult when the haulm is growing particularly quickly.  Rapid growth will be the case when temperatures are high and soil moisture is not limiting during the period of rapid haulm growth. Also, haulm growth will be more rapid for some varieties compared with others. The rate of production of new leaves and growth of existing leaves will be greater for any crops planted later. 

The main choice as far as fungicides are concerned is between using systemic fungicides and non-systemic fungicides. Trials continue to demonstrate the benefit of applications of systemic fungicides. The use of systemic products should ideally start when the leaves are meeting along the drills, i.e. when there is plenty of haulm to intercept the spray. Keep intervals appropriate to the risk.

Trials have demonstrated that a robust programme applied during rapid haulm growth can make a substantial difference not only when the crop is severely challenged by blight during rapid haulm growth but also when blight risk is high during the canopy stable phase of growth.

The threat from blight each year is mainly determined by the prevalence of weather conditions suitable for its spread and development. Assuming weather conditions favour the development of the pathogen the current dominance of the more aggressive genotypes of the blight pathogen means that blight threats are higher compared with a few years ago. The recent mild weather has triggered High risk weather for late blight in some regions, and in these regions, you may still be looking at little haulm growth, but the risk of infection is still there.

1. Blight outbreaks are likely to occur earlier. This is not because outbreaks are being started by soil-borne oospores but because more aggressive genotypes of the pathogen produce spores earlier and in greater numbers. Current evidence suggests that the primary sources of inoculum remain the same, i.e. outgrade piles, groundkeepers and seed tubers.   

2. There is likely to be less scope to extend spray intervals. If there’s any gap in fungicide protection then this is more likely to be exposed than previously. Fungicide programmes must be started early enough and spray intervals must be short enough for the risk conditions. The shorter latent period and ability to produce spores more quickly means that when blight develops it will do so more quickly. Growers should adhere to a maximum 7-day interval unless there are periods of extended low risk.
 
3. For some varieties recent trials have confirmed that resistance ratings obtained when tested using the new, more aggressive strain of blight 13_A2 were lower than the published ratings. There is no doubt that 13_A2 is at least partly responsible for changes in varietal resistance ratings. If growers are relying on cultivar resistance as part of the blight control programme then they should refer to the most recent ratings, derived from tests in the previous three years using 13_A2.   

For optimum blight control it is not sufficient simply to use the most effective products because the decision over which fungicide to apply when can be critical. In recent trials blight was best controlled where the crop received the most effective fungicide, or combination, during periods of greatest risk of infection. At times curative activity and/or tuber protection are required in addition to good protection against foliar blight. For optimum control the blight fungicide programme should be based on a pro-active approach, using the most appropriate fungicide product for the growth stage and the challenge posed by blight.