Favourable wet weather conditions have put downy mildew at the front of this season’s disease control firing line. And with many growers of ornamentals contending with the problem, in some cases for the first time, Dr Martin McPherson of Stockbridge Technology Centre (STC) says that defeating this clever pathogen can be a challenge.
“The cool, and extreme wet weather through June and July have been ideal for downy mildew to thrive, with overhead irrigation and/or condensation forming on the leaves in the early morning increasing the level of leaf surface moisture,” says McPherson. He adds that with rising energy costs creating a tendency to adjust temperatures downwards there is a heightened pressure on fungicide use, but it is equally important to adopt other strategies such as adjusted irrigation timings or improved air circulation to keep the foliage dry.
The environmental and economic factors linked to downy mildew risk are however also being joined by the threat from ‘alien’ pathogens and pests from the greater quantity of plant material sourced from overseas.
McPherson explains that with pathogens like downy mildew there may be a considerable latent period when plants are symptomless, but infected (taking around 10-14 days from initial infection to symptom expression). “This potentially allows both existing (indigenous) and new (non-indiginous) pathogens to arrive in the UK undetected,” he warns. “And in the case of ‘alien’ pathogens (or indeed pests) there is the added threat of statutory intervention and this can potentially be devastating for businesses accidentally caught up in eradication campaigns.”
A recent example is downy mildew in impatiens; a previously unknown disease in the UK until STC first reported it in its Plant Clinic in 2003 where it was thought to have arisen on vegetative cutting material sourced from overseas.
The disease has reappeared in spring 2007, again, thought to have been re-introduced from imported plant material. Fortunately, following initial statutory intervention, impatiens downy mildew was taken off quarantine control and this has allowed the industry to take full responsibility for its control.
Accepting that growers routinely deploy cultural measures for disease control, McPherson explains that growers can support this with either a preventative or an eradicant fungicide strategy.
“Where there is a known risk of downy mildew such as on high risk crops like pansy and viola, nicotiana, antirrhinum and impatiens, it is advisable to apply a routine preventative programme to guard against infection occurring,” he recommends. Spray intervals can be extended by using a systemic product with good persistence that will protect the existing foliage as well as new growth. “Aliette applied as a drench for example, due to its systemic nature, could deliver preventative control for up to three weeks,” notes McPherson.
In other situations where the risk of downy mildew is lower either due to the reduced susceptibility of the host or the prevailing weather patterns, growers may opt to delay fungicide application until the first signs of the disease. Here, it is important to detect the symptoms early, identify the pathogen accurately and intervene with an effective systemic fungicide as quickly as possible. “But the risk is that if those signs aren’t spotted early enough full control may prove to be more difficult.”
A further benefit of the preventative strategy is that growers treating routinely for root pathogens such as Pythium and Phytophthora will gain incidental control of both downy mildew and white blister (Albugo spp.) providing they select a systemic product that continues to provide protection to the new growth.
Resistance issues are a further threat to effective control of downy mildew. McPherson explains that downy mildew pathogens present a high resistance risk making it particularly important to ‘ring the changes’, using products from different chemical groups to make it more difficult for any resistant strains in a given pathogen population to survive.
“The good news is that there have been no reports of resistance to the Certis fungicide Aliette. This is probably due to the mode of action that stimulates the plant to produce phytoalexins, encouraging the plant to defend itself against infection and providing multiple defences against the disease,” he says. Used in conjunction with single site fungicides such as metalaxyl-M, where the resistance risk is potentially greater, and the older multi-site protectant products like mancozeb, effective control ought to be assured. l
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