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Do you know how to distinguish and control Alternaria, or early blight?

30 January 2014
 

Although not in the same league as late blight, it is important to understand the potentially damaging effects of Alternaria

 
At AHDB Potatoes’s recent Winter Forum West in Shropshire, John Keer of Richard Austin Agriculture told a room full of growers and industry what we know about Alternaria and the way in which the two species can affect potato crops. 
 
John advised “For clarity, as both forms of blight can be seen ‘early’ or ‘late’, we should use Alternaria to refer to early blight, as Phytophthora infestans is known as late blight, or just simply blight. Alternaria can be more sporadic than late blight but certain varieties are more susceptible to it, so growers need to know how to identify Alternaria and how to control it.”
 
Whilst less prevalent than late blight, incidents of Alternaria are increasing due to warmer summers, changes in general blight fungicide use and to more susceptible varieties being grown which develop greater levels of inoculum.  “There tends to be a greater incidence of Alternaria in crops under stress, such as water stress or nutrient deficiencies,” John continued. “Also, factors which restrict the rooting system - for example, PCN or compaction - can also increase the incidence of Alternaria.  
 
Alternaria has two species: A. alternata and A. solani. John has found that both species can infect a crop at any point in the season. A. solani will infect healthy leaves, where A. alternata tends to infect stressed or senescing tissue. Although A. solani appears to be the more aggressive species, it is the species that can be controlled by a range of fungicides. The only sure way to establish which form of Alternaria you are dealing with is to have the spores examined under a light microscope. 
 
If a regular pattern is seen on the leaves, this is more likely to be a nutrient deficiency, as Alternaria has no regular pattern and unlike late blight, is restricted by major leaf veins.
 
Growers who suspect their crop is affected by either form of Alternaria need to take action, as yields can be affected by as much as 30% in very severe cases. This is caused largely by the loss of the photosynthetic area on the foliage which causes smaller tubers and irregular bulking, and possibly by lower dry matter. Also there is potential for tuber damage, rendering a crop unmarketable, although this type of damage is extremely rare in GB but is seen on mainland Europe. This type of damage may increase with warmer summers and any subsequent increase in Alternaria inoculum.
 
When considering varietal susceptibility to Alternaria, there are no official ratings although Markies, King Edward and Vivaldi are all thought to be very susceptible.
 
Gary Collins, the technical executive who manages the blight campaigns for AHDB Potatoes said “Recent trials work carried out by John revealed useful information on the contribution that blight fungicides make to Alternaria control. However, due to the unpredictable nature of disease development, there is as yet no consensus on when to start spraying against Alternaria or the intervals needed between spraying.” 
 
“Nevertheless,” Gary advises “it is clear that if you grow a variety susceptible to Alternaria, then you will need to remember to include those activities which will help to control it. There are fungicides for use throughout the growing season that treat Alternaria as well as late blight, but the best late blight fungicides offer no protection against Alternaria. Growers need to check and plan applications carefully, especially during periods of high blight pressure.”
 
Although there are still many unknowns about Alternaria in GB, AHDB Potatoes has produced a new guide called ‘Managing the risk of early blight (Alternaria spp.)’ which gives independent information to growers and agronomists needing to plan their control programmes. 
 
You can view and download the new Guide at www.potato.org.uk/publications or email potatoes.publications@ahdb.org.uk for a printed copy.
 
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