Preparing for harvest – British Potato Review

Last year, he visited the Netherlands to look at a tractor-mounted haulm pulling machine.

“On the front was a topper, which left 20cm stalks, and the device on the back guides the haulm between rubber discs which grip and raise the haulm,” he explains. “Two metal plates are strategically placed so they move across the top of the ridge to keep it in place so the tubers do not come to the surface.”

The challenge is that for this innovative machinery to work effectively, the potato plants need upright haulms, yet breeding programmes are focused on market-driven characteristics rather than agronomic benefits, he said.

Electric haulm destroyers are also being put through their paces to assess their potential efficacy in desiccating potato crops as an alternative to chemical desiccation in ware and seed potatoes.

“These work by spraying the crop with a conductive fluid from a sprayer on the front of the tractor to increase conductivity, and then applying an electrical charge of 5,000 volts from the back,” said Andrew. “This wilts the green leaf area and instigates senescence.”

During the growing season Andrew recommends tissue tests to monitor the crop’s nutritional status. Nonetheless, as it comes to tuber bulking, there are some crops that can benefit from a feed to help the tubers reach their maximum potential, he adds.

“If you choose this strategy, it is really important to apply it between tuber initiation and bulking, because once the plant starts to senesce, the nutrients and energy are going down to the tuber rather than being retained by the haulm.

“Generally speaking, by this time, the tubers should have sufficient nutrients, but there are often hot, dry spells in July, stressing the crop and reducing potential yield. Applications of seaweed products and some biostimulants can alleviate the effects of such stress, and are often more effective than late season nutrient applications.

“Nonetheless, the combination of biostimulants and nutrition keep the haulm greener for longer, optimising harvest potential.”

Decisions to terminate the crop can be decided by the crop reaching its optimum size (or specification demanded by customers), time of year and the need to get the crop into store in good condition.

First, decide your harvesting date, and then count back three – or even four – weeks, he advises, adding that some varieties, particularly indeterminate ones, can take even longer.

“Success hinges on planning, timing, correct choice of your method of haulm destruction and environmental conditions at the time. The idea is to decide exactly when we want to stop the tubers bulking and stimulate skinset.

“Haulm destruction strategies are best devised before planting, deciding on aspects such as nitrogen management, whether the crop is destined to be a seed or ware, determinacy and, for ware and processing crops, expected storage time.”

Desiccation is best done at the start of senescence, he says, adding that seed crops need to be terminated earlier so tuber size is optimal. However, there can be a risk that early flailing can shock the crop and can give rise to vascular browning.

“If possible, apply a desiccant to slow the crop down and make an artificial start to senescence, rather than going in to flail a green crop that is still growing vigorously,” he advises.

“Desiccation works best when the crop is already senescing, so some growers have been reducing N applications so the crop starts to senesce naturally.” For example, when devising nutrition strategies for seed crops, applying 100-110kg/ha/N rather than the 185-200kg/ha/N for ware.

Andrew emphasises the importance of leaving enough time between desiccation and lifting date, to get the best quality crop out of the ground.

“Good skin set reduces the risk of losses and rejections from bruising, as well as minimising susceptibility to mechanical damage which provides a pathway for bacteria and fungi to enter the crop.

“It also gives more time for the stolon to become detached from the tuber, reducing damage and making for more efficient lifting.”

He adds that growers keep seed crops in the ground for as little as possible, because it results in a lower physiological age, as well as reducing skin disease and minimising the risk of aphids landing on the crop and vectoring virus.

Reflecting on the wet harvest of 2023, he notes that if growers are worried about the damage that will be done at lifting, early desiccation can help the ground dry by increasing the airflow and light onto the ridges.

Varietal determinacy also plays a key role because of its influence on haulm size, vigour and bulk, making such varieties more difficult to stop, notes Andrew, adding that later maturing indeterminate varieties can be more difficult to flail effectively to an even length, because the haulm has often dropped down into the valley of the ridge.

Storage requirements are a critical element. Crops destined for long-term storage, such as processing varieties, need to go into store in the best conditions possible.

“We have to remember that a store is not a hospital, and potatoes never come out in a better state than they went in,” said Andrew.

There are a number of different strategies, including applying a desiccant spray to the haulm before flailing, or going in straight with a flail and then finishing off with a desiccant (or two, if necessary).

When desiccating, Andrew observes that the actives Carfentrazone-ethyl and Pyraflufen-ethyl can be used in various combinations to effect good haulm control.

He also reminds growers to be aware that both have a maximum seasonal limit of 1.6l/ha, and ensure that they maintain the relevant harvest intervals. (Carfentrazone-ethyl has a seven-day harvest interval, while for Pyraflufen-ethyl it is 14 days.)

Water rates need to be at least 300l, otherwise efficacy is severely reduced, so cutting back to increase speed of application is a false economy.

Moving on to talk about nozzles, he notes that while both flat fan and forward-back nozzles can work well, but he prefers the Defy nozzle, or, now, the new Syngenta 3D90 nozzle.

“The weather at this time has a big impact, and haulm destruction is always better in sunny conditions rather than when it is cloudy and damp, so if the weather is poor is often better to wait.”

Because of the impact of the sun’s rays, spraying should preferably be done between mid-morning and late afternoon on a sunny day.

“If you are working to a tight time schedule, and waiting for better weather is not an option, flailing is better than spraying,” Andrew said, but added that growers should not flail when blight or blackleg are present as this encourages disease spread.

Before flailing, the crop should first be opened up with a desiccant and left for seven days before going in with the topper, Andrew said.

“Ensuring attention to detail when setting up the topper is key to a good result; blades need to be sharp and the flail should be set to cut between 15 and 20cm above the ridge top. It is not easy to get the right length because it depends to a certain extent on variations in senescence, and wider toppers can be more difficult to manage because of land contour differences.

“If you go too fast, you do not create the vacuum which pulls the haulm up and this results in less effective flailing.”

“It is always possible to tell when there has been insufficient attention to detail by the way the haulm dies,” he said.

If the crop is indeterminate with a lush canopy before flailing, a further desiccant may be needed, he adds, noting that this can between 24 and 48 hours later so the haulm at the top of the ridge has time to dry off.

After flailing, Andrew always recommends applying a blight spray with tuber blight activity, and in seed crops the addition of an insecticide can reduce the risk of aphids vectoring virus into the crop.

Skinset or total yield: What’s the bottom line?

If nematicides were applied before planting, Andrew reminds growers that the harvest interval needs to be complied with, but other than that, deciding when to stop the crop remains an area for much discussion between grower, agronomist and processor.

“Is it better to hold on for another week and perhaps get another 2t/ha, but leave yourself open to higher rates of rejection from bruising, and, in stored crops, more rots if the weather at lifting turns wet?

“Skinset is crucial to helping the crop cope with adverse harvesting conditions, so delaying raises the risk of losses. Moreover, you have no yield until the crop has been lifted and gone into store, and the revenue collected from selling the crop is the most important factor.

“You have to ask yourself, is it worth the risk of holding back?”