Crop Clinic

Late blight is caused by the fungus Phytophtora infestans  and it first became a major problem in Europe in the 18th Century, where it led to extensive destruction of potato crops in Ireland.

Although seed and potato volunteers are potential sources of the disease, the key source are potato dumps. A potato dump is a pile of potato tubers which have been discarded in previous years.  These dumps are a concentration of diseased tubers which, if ignored, sprout early in the year and allow the disease to develop. 

Typical disease symptoms comprise pale brown or black watersoaked spots on leaves.   The white fungal growth of the fungus is generally seen in the zone between the lesion and healthy plant tissue.  This is because late blight is a biotroph and lives on healthy green tissue. Lesions can also develop on the stems and appear as pale brown watersoaked marks.  The spores which develop from the fungus fom these lesions are dispersed in the wind.  These spores may infect more potato plants  directly, or produce swimming spores known as zoospores which infect more plants.  Heavy rain can wash these spores from the leaves and stems onto the ground where they can infect the tubers.  Symptoms on the tubers comprise brown mottled lesions can be seen inside the tubers close to the surface. In severe cases, the marks can be seen without cutting open tubers.

Recent changes to the GB blight population
The most recent information from the Potato Council-funded project on the UK  blight population is that the population is now dominated by two genotypes of P. infestans, i.e. genotypes 13_A2 and 6_A1, which in 2008 comprised 79% and 12% of the population respectively. Genotype 6_A1 is of similar aggressiveness to 13_A2 therefore most of the comments in 1 and 2 below regarding 13_A2 also apply to 6_A1.

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 presence of 13_A2 means that:

1. Blight outbreaks are likely to occur earlier because 13_A2 will be more active at lower temperatures than most other genotypes. In cooler conditions 13_A2 produces larger lesions that sporulate sooner than other genotypes. The 13_A2 genotype also spreads quickly.

2. There is likely to be less scope to extend spray intervals because 13_A2 has a shorter latent period between leaf infection and the production of new spores. This means that the pathogen will complete its life cycle in less time than previously. 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 7-day intervals unless there are periods of extended low risk.
 
3. In the last few years comments were made about some varieties appearing to be no longer as resistant to foliar blight as their published resistance ratings. Recent trials, some funded by the Potato Council, have confirmed that for some varieties resistance ratings when tested against 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. However, further testing is required before sound conclusions can be reached. It is important for growers to refer to the most recent ratings, derived from tests in 2007 and 2008 using 13_A2.   

Risk of oospore-derived outbreaks
The situation for the last few years has been that some blight outbreaks in crops, on volunteers and even in outgrade piles, have contained both the A1 and A2 mating types of P. infestans. Consequently it is possible that oospores are present in fields that are going back into ware potatoes this year and the risk of blight outbreaks initiated by oospores is higher than before. The use of short rotations increases the risk considerably where the previous potato crop or volunteers were blighted and both the A1 and A2 mating types necessary for the production of oospores were detected. The production of oospores can be minimised by dealing very quickly and effectively with blight outbreaks and the risk of oospores initiating outbreaks can be reduced by minimising volunteers as much as possible and avoiding short rotations.

Ensure there is no growth on discarded dumps of potatoes. Covering the heaps with black polythene is one method to prevent growth.

All crops should be protected with fungicide when a High Risk period is triggered ,or as soon as the haulm meets in a drill. High Risk Smith Periods are defined as a period of two consecutive days where  the minimum temerature remains at is 10C over the whole  48 hour period and where the relative humidity remains above 90% for at least  11 hours on each day.