Advice on reducing risk of late season disease in potatoes

With unsettled weather set to continue, potato haulm destruction could be a protracted process in many crops, particularly indeterminate varieties or those grown for seed.

SAC Consulting’s potato expert Kyran Maloney warns this could significantly heighten the risk of late season disease and threaten crop quality.

See also: How alternative potato desiccation systems performed in 2022

He advises growers to identify the potential risks in each individual crop and tailor ongoing haulm destruction and harvest strategies to reduce the impact of disease on marketable yield.

Seasonal risks

In a dull and wet season, the chances of a long period between haulm destruction and harvest are increased, exposing developed tubers to plenty of disease inoculum.

Top of the list are the bacteria that cause blackleg and soft rotting – the main one being Pectobacterium atrosepticum – which are always more prevalent in cool and moist conditions.

Another potential red flag is shorter stolons in some varieties, which are a result of the dry start followed by wet.

A similar phenomenon occurred last year. Set tubers are closer to mother tubers breaking down due to soft rot, or diseases such as gangrene and black dot that colonise stems and move down stolons to infect the crop.

Considering these risks, Kyran says growers should avoid the temptation to leave any crops in the ground to bulk longer than is necessary, because in a wet year it will provide diminishing returns.

“Every day you delay, days get shorter and there is less potential for bulking anyway. At the same time, disease risk increases and what you might gain in bulk, you lose throwing out as brock [waste] at grading.

“Get a feel for what is out there in crops during test digs, and if there are any issues with soft rots or other diseases, prioritise those fields for earlier desiccation and harvest,” he says.

Destruction advice

Most growers will be using a flail and spray system to stop crops and trigger skin set, and most will have a good handle on strategy in the third season since the withdrawal of diquat.

Kyran says the only factor which might require a change in tactics is the presence of disease, with Scottish rouging teams reporting the odd late blight lesion in seed crops in recent weeks.

“If there is blight or blackleg in a crop, consider taking an all-chemical approach instead of flail and spray to avoid spreading disease inoculum,” he says.

“However, bear in mind that PPO [protoporphyrinogen oxidase] inhibitors alone can take more time to achieve complete haulm destruction, particularly in seed crops or big, indeterminate varieties. That increases risk of the other diseases like black dot and gangrene.”

When flailing, Kyran reminds growers to leave at least 15-20cm of stem above the ridge to ensure there is enough of a target to cover with desiccant.

Coverage when applying contact-acting PPO inhibitors is crucial for optimising efficacy. If spraying direct to a full canopy, water of 300-400 litres/ha is recommended, while 300 litres/ha should be used when applying post-flail.

“It might seem excessive when targeting stems, but getting good coverage improves the chances of killing the buds on the leaf axials and reduces the likelihood of regrowth.

“We’ve also done some work at SAC on nozzle choice. Angled or twin-fan options can improve coverage, particularly when aiming at a vertical target like a potato stem,” he explains.

Field hygiene

Where using the flail, Kyran reminds growers that field hygiene could be an important factor in minimising spread of blackleg or late blight between crops.

Plan to flail them last in the day and clean down machinery, or clean down machinery between problem crops and healthy ones.

“It’s particularly important for seed growers that have young, early generation material that has been kept clean all season. You don’t want to be bring in contaminated material at such a late stage,” he warns.

Prevent regrowth

The potential for regrowth is high this season, with plenty of moisture and nutrients available to crops.

This can cause slow skin set, which leaves tubers vulnerable to infection by bacterium, late blight and other disease such as gangrene and black dot.

At harvest, if crops are lifted before adequate skin set has been achieved, it elevates the risk of physical damage, which again opens the door to infection.

The second danger is the regrowth getting infected with late blight, which in turn creates a risk that zoospores will get washed into the ridges and onto tubers.

Compounding that risk is the fact that some crops were forced into cloddy seed-beds in the spring, which makes the path of spores to tubers much easier.

“Fungicides with tuber blight activity should be used from the start of haulm destruction, right up until all haulm is dead, because even low levels of late blight on green foliage or stems poses a risk to tubers.

“There has been a lot of money spent on keeping blight out of crops up to this point, and growers don’t want to fall at the final hurdle.”

Regrowth can also act as a route in for virus, with aphid vectors landing on any green areas and potentially infecting crops with PVY variants or potato leaf roll virus (PLRV), which have been seen widely in UK crops this year.

For seed growers, virus management is essential, and Kyran says aphid activity should be monitored and control programmes continued until all green stem and leaf area is dead.

However, he accepts there are practical limitations to which insecticides growers can use.

Resistance to pyrethroids is common in some aphid vector species, and the two main systemic products, Teppeki (flonicamid) and InSyst (acetamiprid) are limited to two application each. These are likely to be used up by the start of haulm destruction.

That leaves Movento (spirotetramat), but its cost is much higher than other options.

“Monitor crops carefully for signs of regrowth and focus any treatment on those areas,” he says.

“It’s more important to ensure a good canopy kill in the first place, which should help avoid regrowth and need for further blight fungicide and insecticide use.”

Seed treatments

A consideration for seed growers is the need for fungicide seed treatments before crops are loaded into store, with dry rot and gangrene the two key targets for products such as Gavel (imazalil) and Storite Excel (thiabendazole).

Dry rot favours hot and dry conditions where tubers are susceptible to damage at harvest, allowing Fusarium into the tuber.

While this season looks like being cool and damp, there could still be a risk of damage where crops are harvested before adequate skin set.

“Gangrene develops in store later, so you can only judge risk by conditions,” says Kyran. “Risk is increased in wet soils and a big gap between burndown and harvest.

“Gavel and Storite Excel can be used to protect seed crops from dry rot and gangrene and are best used as soon as possible after harvest, using a hooded applicator over a roller table.”

Delayed lifting

There are plenty of disease risks that can be negated by rapid haulm destruction and lifting, but one situation where it may pay to leave crops in the ground is where there is evidence of soft rotting or a few blighted tubers.

Kyran says lifting these crops a bit later can allow the rots to break down completely before going over the separation system of the harvester.

“That will reduce the spread of inoculum during the harvest process, and the likelihood of rots making it into boxes and causing problems later in store.

“However, it doesn’t always work if the weather is not favourable, so I’d encourage growers to keep digging and respond to what they find.”