Plan to manage alternaria threat

Potato crop fungicide programmes will need to be adapted this year to protect crops against alternaria as well as late blight, according to Neil Thompson, product manager for potato fungicides at Bayer CropScience.

Alternaria, or early blight has become an increasing problem over the last two years, probably as a result of growing popularity of more susceptible varieties and changes in fungicide use, he says.

“Limitation of mancozeb use has contributed and there has been a knock-on effect from the development of more aggressive late blight strains. Recently-introduced fungicides tackle them very effectively but have little or no alternaria activity.”

So far, the variety Markies has been worst affected while Saturna and Maris Piper have shown susceptibility too. And although disease outbreaks have generally been seen from late July onwards, primary infection can be spotted much earlier.

“Initial symptoms can be found two to four weeks from emergence on older leaves lower down plants. Look for irregular shaped dark brown to black spots. These develop into characteristic ‘target spot’ lesions, with dark concentric rings surrounded by a chlorotic halo.”

Agrovista technical manager Dr Mark Palmer advises adaptation of fungicide programmes for susceptible varieties to build in early protection against this primary infection. “The key thing is to keep ahead of alternaria risk by using protectant products with dual activity against alternaria and late blight.”

He advises use of Consento (propamocarb+fenamidone) during the rapid canopy growth phase.

“Its propamocarb component provides well-proven systemic mobility needed to protect new growth against late blight while its fenamidone component adds good alternaria activity.”

Dual activity

Mancozeb is another dual activity option when used at rates over 1,500g/ha, but today’s mancozeb containing co-formulations deliver less than this so are unlikely to have much effect against alternaria, says Dr Palmer.

“From canopy complete onwards, susceptible varieties still need to be protected against secondary alternaria infection but the focus must be on late blight control. This is the time when programmes need to switch to the premier league late

blight products - Infinito (fluopicolide + propamocarb), Ranman (cyazofamid) and Revus (mandipropamid) - to combat the more aggressive strains now dominating.

“As these are specific late blight products, should periods of high alternaria risk occur - alternation of warm and dry weather with short periods of high moisture - additional alternaria specific treatments will need to be slotted into the programme.”

Currently the only option is Olympus (azoxystrobin + chlorothalonil) which has a SOLA for this purpose but Signum (boscalid + pyraclostrobin) may soon become an option too, adds Dr Palmer. A SOLA for Signum has been applied for but is not yet approved.

Alternaria symptoms are quite different to those of late blight, says Masstock technical support manager Malcolm Smith.

“But distinguishing which of the two species of alternaria you have in your crop, be it alternaria alternata or alternaria solani, is nigh on impossible in the field,” he says.

Markies and Saturna were strongly affected in 2009. However varietal resistance to alternaria in the main UK potato varieties isn’t well characterised, so growing specific varieties is unlikely to offer a significant control measure, adds Mr Smith.

Trigger

“The real trigger for last year’s infections was the hot spell we had back in June.

“Temperature is the main factor. High temperatures really favour its development - which is likely to be why it’s not normally so much of a problem here in the UK,” he says.

Alternating periods of wet and dry conditions are also favourable to spore germination and the spread of infection.

But even with the possible risk from alternaria, Mr Smith reminds growers that anything could happen during the coming months in terms of disease. “Alternaria does pose a consideration for control but growers preparing their programmes really must ensure the focus remains on late blight,” he says.

“I am a keen supporter of the newer chemistry such as Valbon (benthiavalicarb+mancozeb), Revus, Ranman and Electis (mancozeb+zoxamide).

“Growers should be planning their programme, taking into consideration the various attributes of the products, and ensuring they have a robust strategy incorporating a range of different modes of action.”

“By no means do we have the whole answer, but the inclusion of mancozeb co-formulations can provide additional advantages to alternaria control as well as providing a robust late blight strategy.

“Valbon (benthiavalicarb and mancozeb) is one option that has a fit early on in the programme. Timed best during the full stable canopy stage it provides some much needed kick-back and protectant activity against foliar blight.

“Mancozeb is a very useful active with a reasonable level of activity on alternaria,” says Mr Smith. Its use in co-formulations is useful in giving late blight treatments activity against alternaria, but is also beneficial for anti-resistance strategies by providing multi-site activity against foliar late blight.

While alternaria has become more of a problem in the UK, it is always alongside late blight and is rarely found in isolation, says Dr David Stormonth, technical manager for Interfarm.

The protectant fungicide mancozeb is regarded as a key active against alternaria on the Continent where alternaria is a significant problem and the newly re-registered Dithane (mancozeb) now has a label recommendation for early blight in potatoes in the UK, he says.

“There is some view that the decline in the use of mancozeb as the mainstay active in blight programmes could have some impact on the reported increased risk of alternaria, as most other blight fungicides, particularly the newer chemistry, have little or no activity on this disease,” adds Dr Stormonth.

It makes no sense to apply a product for alternaria alone, but it does make sense to incorporate mancozeb into programmes to keep both early and late blight under control, particularly if growing susceptible varieties, he adds.

The continental climate favours alternaria over late blight, says John Edmonds, European technical manager for Gowan Comércio.

Teperatures

“If our temperatures continue to rise, as they are expected to do, we are likely to see more alternaria in our crops. Realistically applying a fungicide solely for the control of alternaria is just not practical or indeed cost-effective in the UK, however. You need a fungicide that has excellent and reliable activity against both late and early blight,” he says.

Mr Edmonds points out Electis does not yet have label approval for alternaria in the UK but it does have full label approval in some European countries, including France and the Netherlands, where alternaria has been a significant problem for many years.

Have your say

Please do not complete the following form field for security reasons.

Name

E-mail

Comment

Post as

Anonymously

Display name