Potato Review

36 POTATO REVIEW MAY/JUNE 2020 MH: FOLIAR APPLICATION ➜ He estimates 60% to 70% of Dutch growers use it each season and this is only set to increase as they are forced to live without CIPC. In his experience, application timing is the overriding success factor because of the way the active works. It must be applied to a fully functioning plant and given time to be taken up by the leaves and translocated to the tubers. is means that a plant must not be su ering stress from heat, drought or disease and the product must be applied long before the planned date of desiccation to optimise uptake into the tubers. “You should apply between three and ve weeks before desiccation, with the optimum being ve weeks,” advises Fokke. Independent potato consultant Martyn Cox agrees that timing trumps all other factors in getting the most out of MH and emphasises that conditions at application must be within the recommended environmental parameters. Relative humidity With uptake intrinsically linked to relative humidity, he says making sure the product is sprayed when RH is above 75% and not likely to fall for a considerable period afterwards is critical to avoid compromising uptake. It should also be applied below 25C when no rain is forecast, or irrigation scheduled for at least 24 hours after spraying. “On warm and dry days, it will be better to spray in the evening when its cooled and ahead of the dew point. If you have to wait a bit longer for the correct conditions, that’s the right thing to do, as uptake is the most important factor for e cacy. Don’t blindly stick to the ve-week limit.” Fokke says in a normal season hitting the three to ve-week pre-desiccation window should be achievable and also advises using higher water volumes with a slow forward speed. “Where you use less water and travel too fast, deposition is poor, and the solution dries too fast on the leaf and hinders uptake. In trials we see 250-litres/ha as the minimum, but we recommend 300 – 500 L/ha where possible, particularly if the weather is warm and dry.” Balancing act Growers should be well-drilled in planning desiccation timing and some will even have a date in mind before planting. Martyn says in these situations, counting back 35 days from the planned desiccation date provides a reasonable guide to MH timing. However, where it becomes more of a balancing act is on varieties that are highly determinate, such as Innovator. ey can senesce very quickly, particularly on a site with low nutrient status or where the crop is stressed by heat, drought or diseases such as Alternaria spp. Determinate varieties should be prioritised for inspection about six weeks from the expected burn o date and MH applied when conditions are optimal. Otherwise, growers may get caught out as MH cannot be applied to a senescing crop. “For indeterminate varieties that go on forever, like Cara or Markies, you cannot be complacent, but you have more breathing space. “Once owers have dropped o – if the variety does ower – and you have the majority of tubers close to the desired size fraction (80% greater than 25mm), then you can think about spraying at the next opportunity,” said Martyn. Standalone product Blight fungicides can contain adjuvants that help the product stick to the leaf and dry faster but this is counterproductive to maleic hydrazide uptake so it should be applied alone. “I’m often asked if you can add an adjuvant or sticker to maleic hydrazide in case of rain, One of several benefits of applying maleic hydrazide includes a reduction in secondary tuber growth, which was widespread in 2018 and 2019. Photo: Blackthorn Arable