Plan Early for Successful Potato Late Blight Control
May 21, 2020
Robin Bolton, CAFRE Senior Crops Development Adviser
Blight is a constant threat and significant cost to local potato growers. Growers should put plans in place early with their Basis agronomist to ensure a proactive approach is taken to prevent blight. Controlling this disease is becoming more difficult each season as blight fungicide active ingredients are withdrawn from the market and the disease produces new strains which are resistant or less sensitive to some of the current chemistry.
Results from Euroblight monitoring for 2019 would suggest that the new 37A2 strain of blight is now widely established in Northern Ireland and as such blight control from Fluazinam (Shirlan) can no longer be relied upon. Also the 36 A2 was also detected in N Ireland and this strain is thought to be more aggressive.
New active ingredients have come to the market, many of which have added to the ability to control blight. However with more widespread sexual reproduction the disease now has an increased ability to create new strains and if these new products are not used carefully resistance could develop very quickly.
To put less pressure on the fungicides it is important that good Integrated Pest Management principles are applied for blight control. The starting point is following a good rotation and growing a resistant variety from high grade seed stocks. With most crops now drilled these variables are already set. The next step is to ensure sources of blight are minimised. Potatoes growing in out-grade piles need to be controlled to reduce the source of inoculum. Local fields that previously grew potatoes should be inspected for the presence of volunteer potatoes and where possible these should be controlled too. Potatoes grown in gardens can also pose a threat and where possible taking the time to have a positive conversation with gardeners about the risk blight in their plots can pose to your commercial crop may be beneficial.
The most important thing when spraying to prevent blight is to start your programme early. Most blight fungicides available on the market have little or no curative activity so it is vital that the crop is protected from the start. First blight sprays should be targeted at the rosette stage of the crop and the crops then need to be treated at regular intervals depending on blight pressure. On line alert systems such as AHDB Blight watch will issue warnings based on your local postcode to help you judge the disease pressure.
The first Hutton period, indicating favourable weather conditions for blight, this year was issued on 19th May and the advice would be to have any emerged crops sprayed with fungicide. In the early stage of growth up to canopy complete, systemic and translaminar fungicides should be used to ensure new growth is protected. To avoid resistance developing label conditions should be adhered to and products should be rotated based on their mode of action following FRAG guidelines. Crops should be inspected frequently during the growing season and if any blight appears the affected plants should be desiccated and removed. Samples of the blight should be sent for analysis to determine the strain of blight present your local CAFRE crops adviser or Forest Service plant health inspectors can arrange this.
Another key aspect of a good blight control programme is to ensure spraying is continued until the halum is completely dead. Bear in mind that without Diquat in 2020, chemical desiccation may take longer and therefore crops need to be protected for longer. It is also important to ensure that you use a fungicide with good Zoospore activity for the final blight sprays during crop desiccation. It is important to plan this early in the season with your Basis agronomist to ensure you have enough of these fungicides left to use during desiccation within label constraints.
For any further advice on controlling late potato blight consult your local CAFRE Crops Development Adviser or local Basis trained agronomist.