Sponsored by Bayer Cropscience

Alternaria is becoming more of a problem in UK potato crops, and can be hard to spot, writes UAP potato specialist Barrie Florendine

All potato growers are familiar with late blight, Phytophthora infestans – one of the most devastating crop diseases in agriculture. Alternaria, also known as early blight, can infect potatoes throughout the season, although infections two to four weeks after emergence are generally worse.

Although its impact on the crop is not as severe as P infestans, it’s difficult to diagnose. What’s more, recent changes in blight control programmes make it a disease worth looking out for.

What is Alternaria?

Alternaria is a fungal pathogen, mainly soil-borne, that affects potato crops. It is a global disease that has been present in UK crops for at least 30 years. If left unchecked, it can cause sudden and dramatic leaf loss, leading to yield penalties of up to 30%.

Although it is not a new disease, to date there has been very little research undertaken into the disease in the UK. So most practical advice is based on research carried out overseas and observations in UK crops.

The predominant species is Alternaria solani. Another species that affects potatoes and comes in slightly later is Alternaria alternata, and it is almost impossible to distinguish the two.

Alternaria causes lesions on the leaves. These usually (but not always) appear a few weeks after emergence and start as very small black or brown spots on lower leaves which then coalesce. This causes the leaf tissue to die as the disease spreads.

Alternaria has been increasingly found in susceptible potato crops.

How do you identify it?

Early and correct diagnosis of alternaria is critical to its control. The problem with the disease is that it is frequently misdiagnosed.

Unlike late blight, you cannot see the spores in the leaves, even with a hand lens – there’s no distinctive milky ring of sporulation around the lesion.

This makes the symptoms look remarkably similar to magnesium deficiency. It is also often confused with manganese toxicity and later on in the season even aphid damage.

It is usually the older, lower leaves that show symptoms of early season infection, because spores are picked up from the soil. Again, this is similar to magnesium deficiency, but the spotting will develop into concentric rings of dying tissue, so it will look different as the disease develops.

How does the disease develop and spread?

Alternaria does not spread as far or as wide as late blight. Like potato cyst nematode it can be transferred on seed or via cultivation equipment, although this is not its primary method of spread.

Between crops it survives on potato or other host debris in the field as mycelium (fungal hyphae) on leaf debris or directly in the soil as spores. These can then infect potato leaf tissue as the crop develops.

Lower leaves of the growing crop pick up spores from the soil, while later season infections from rain splash or wind-blown spores can be spread around the plant.

Periods of wet and dry will favour alternaria’s cyclical life cycle and spur on spore germination and release.

This means conditions in irrigated crops are ideal for disease development. It will thrive under the crop canopy, often developing out of sight, making regular crop inspections and awareness important.

Which varieties are susceptible?

Varietal resistance is a major factor and, if you are growing a susceptible cultivar, you should be on the look-out for the disease. Alternaria-resistance ratings for UK-grown varieties are not available.

But the table (left) gives an idea of which varieties are at risk. This has been assessed by using ratings given in the European Cultivated Potato Database for varieties similar to their UK-grown counterparts – most would be moderately susceptible.

Why is it becoming a problem?

Recent popularity of susceptible varieties, such as Markies, has made alternaria more of an issue in the UK crop. Warmer conditions also favour its development, which means it’s likely to become more of an issue as climate change takes hold.

The disease will also thrive in a stressed crop. This has particular importance if you hold back on N inputs. This may be another reason why Markies is so susceptible. As an indeterminate variety, nitrogen applications are used to manipulate crop development, which can put the crop through periods of stress.

The biggest contributing factor to the rise in prevalence of alternaria is likely to be the choice of fungicide strategy for late blight. Mancozeb is known to have a useful effect against the disease, and there has been a huge reduction in its use in recent years.

How is it best controlled?

Cultural methods of control are limited as its survival does not depend on green material as heavily as late blight.

Controlling debris in the field will help. Good weed management, removing volunteers and alternative hosts such as nightshade, will also reduce levels of alternaria.

Severe cases in other countries have been linked to frequent cropping. This means the long crop rotations, more usual in the UK, limit its severity.

The impact will also be lessened if you don’t let your crop become stressed. However, alternaria control is rarely a factor when deciding nitrogen strategy, for example.

The most important factor when deciding your fungicide strategy remains effective control of late blight.

Decide product choice, dose rate and interval to ensure you have a robust strategy in place. Beyond that, look at add-ons or alternatives that will keep alternaria in check.

If mancozeb is included, at rates of about 1500g an application, this may well be enough to keep alternaria under control. Alternative actives such as fenamidone, if used early on, are also effective against infections.

Most commonly these materials will be found in co-formulated fungicides, and they will secure your alternaria control almost as a by-product of your late blight programme.

But if none of these are included in your strategy, the pressure may be on to use mancozeb along with the products being used for blight control, especially if you are growing a susceptible variety such as Markies.

Another option in high-risk situations is to use a strobilurin-based product, such as Olympus (azoxystrobin + chlorothalonil) that has a specific off-label approval and is active against alternaria. A SOLA application is also pending for Signum (pyraclostrobin + boscalid).