Potato crops face challenges – Farmers Guardian

Dry weather may have kept the risk of late blight low, but concerns are mounting for unirrigated crops. Farmers Guardian catches up with two Hutchinsons agronomists to gauge the latest situation

After just 20mm of rain in May and 40mm in June, crops on lighter ground are feeling the effects of drought stress, and there is concern that yields and tuber size will be reduced.

Those on heavier, moisture- retentive soils, or where there is irrigation, look much better, with many crops entering the stable canopy phase at time of writing.

Rainfall during tuber initiation will be key to determining yield prospects. Seaweed extract- based biostimulants have been a useful tool in helping to alleviate stress – although the proof will only really come at harvest.

Typically, most nutrition would be done by now, but with many crops struggling to take up nutrients in dry soil, foliar top-ups could be useful. One particular issue to watch in this area is magnesium. Although many soils have a high-magnesium index, it is often unavailable to plants. Deficiency symptoms can resemble alternaria, but tissue or sap analysis can confirm it – a foliar application is a relatively easy and cost-effective solution.

Aphids have been a big issue, with many crops hitting treatment threshold earlier than normal in already water-stressed crops. Aphid feeding damage can cause significant issues with crop potential and yield.

Product supply has also been a challenge, complicating control decisions. It is important to continue monitoring aphid activity, and to consider a follow-up treatment, although care should be taken in order to limit the risks to beneficial insects when crops are flowering. Flupyradifurone may be one option, given its better selectivity to beneficials.

As elsewhere, late blight has not yet been an issue in dry weather,ded with more focus on alternaria.

Mancozeb plus cymoxanil can be a good option, but in more susceptible varieties, such as Melody, Markies, or King Edward, consider adding difenoconazole to programmes.

There are some big crop variations across this region.

Those with irrigation generally look well, with good canopies and root systems that are able to access soil nutrients more effectively with extra moisture.

Unirrigated crops have struggled, leading to smaller canopies and some high water deficits. Petiole testing indicates some may be unable to access soil nutrients, something we continue to monitor, with further testing in the next couple of weeks, treating with foliar nutrition if and when required.

With tubers starting to bulk up, it remains to be seen what impact this dry weather will have on yields and tuber size.

An option some growers are adopting to help relieve heat stress is biostimulants, which help to maintain plant structure and counteract oxidative stress.

This allows potatoes to continue photosynthesising efficiently, moving carbohydrate into roots and tubers to maintain yield.

Up to the time of writing, at least, late blight pressure has been low in most crops, although risk remains higher where irrigation is used, and can quickly increase with rain and humidity.

The focus has been more on alternaria, a disease exacerbated by dry, hot conditions. Controls centre on difenoconazole + mandipropamid, fluopyram + prothioconazole, or mancozeb, where stocks are available to use up before the November 30 cutoff. The weather can change quickly, so alternaria controls need to be used in conjunction with late blight fungicides to keep crops fully protected, and always tailor programmes to individual situations.

If hot, dry weather persists, growers planning to apply maleic hydrazide (MH) to help with sprout suppression in store must think carefully before treating crops, as applications should be avoided in high temperatures or where plants are suffering water stress.

MH is ideally applied five weeks before desiccation, so consider whether crops are under stress, the soil water availability, what plans are for storage in the short, medium and long term, and check whether you have the ability to use sprout suppressant products in store.