Monitoring essential as aphid pressure high in potato crops – Farmers Guardian – Cam Murray, Andrew Goodinson

Cropping Profile: salad potatoes, first and second earlies and maincrop

Blight risk

With canopies growing rapidly, blight risk is now high, so crops must be protected with robust, systemic chemistry. An effective approach is two applications of oxathiapiprolin (as in Zorvec) at 10-day intervals, followed by fluopicolide + propamocarb, mandipropamid, or cyazofamid-based products once canopy growth stabilises, depending on disease risk.

Last year we saw a spike in blight risk during July that required a return to more robust chemistry, so it is something to watch out for again.

Aphid pressure is very high, so vigilance and field monitoring is essential, especially for the peach potato aphid (Myzus persicae). Seed crops must be protected from any potential virus transmission, but aphid feeding can also damage ware crops.

Aphids generally migrate into crops from boundary hedgerows and margins, so check these areas first. Chemistry is limited given widespread pyrethroid resistance in Myzus persicae and the withdrawal of thiacloprid and flonicamid. However, one well-timed application of an acetamiprid-based product once treatment threshold is reached should be sufficient to protect ware potatoes.

Cropping Profile: maincrop, 70 per cent processing, 30 per cent packing

Products based on fluopicolide + propamocarb or oxathiapiprolin are the main options in high-risk situations. The new combination of benthiavalicarb + oxathiapiprolin offers a useful step-up in activity where required. Mandipropamid + cymoxanil also works well, while it may be possible to switch to more traditional chemistry if disease threat allows.

With all blight sprays, good efficacy relies on correct application technique. In particular, keep forward speed below 12kph and use air-assisted, angled nozzles to maximise leaf coverage.

Flowering is a key period for managing alternaria risk in susceptible varieties; something exacerbated by plant stress (water or nutrients). Including products based on mancozeb and difenoconazole is worthwhile.

Sclerotinia is another risk to manage, although this is more challenging given issues with fluazinam-resistant blight strains.

Several crops have shown signs of rhizoctonia or free-living eelworms. Symptoms are very similar, but both are generally associated with tighter rotations.

Foliar feeds or biostimulants applied earlier this month will hopefully help mitigate any impact.

Fine tune

With nutrition programmes largely complete, it may be worth conducting a final tissue test around late June/early July to see how effective strategies have been. Although not routine practice, it could help fine tune future nutrient programmes.

Finally, the CIPC ban means many growers will rely on maleic hydrazide (Fazor) for sprout suppression in stored crops. While the label states a latest application timing of 21 days before harvest, we have found optimal results from applying it to a healthy, green crop five weeks before.