With more aggressive blight strains that have a shorter 3 or 4 day life cycle and higher spore production, stronger fungicides that can hit blight at every stage are required. Paul Goddard of BASF says that now Invader (dimethomorph and mancozeb) has a robust higher rate of 2.4 kg/ha, it has just about everything you could ask for in a modern blight fungicide and integrates well into any blight programme.
“Within the potato crop you will have blight at different stages of its life cycle, so you need a product that is active at all stages. Invader has fantastic anti-sporulant activity which has become really important in today’s fight against late blight. It has locally systemic activity, moving within the stem to control blight. Stem blight produces a lot of spores and can be obscured by the haulm. Dimethomorph’s systemic activity will also protect new growth in the leaves and its translaminar activity will protect both upper and lower surfaces of leaves. It also has good curative activity. Multi-site mancozeb has strong protectant activity, with redistribution properties on the leaves. So these two actives combined together give a fungicide that really has it all and can attack blight at so many points in its life cycle,” says Paul.
Eric Anderson of Scottish Agronomy says that two blight strains, 6_A1 and 13_A2, dominate late blight populations and have done so since 2005. “There are subtle differences between these two, such as the 13_A2 is resistant to phenylamides and there is more 6_A1 as a percentage in Scotland, but from the practical perspective, farmers just need to treat them as aggressive blight. So what makes these strains so aggressive? Firstly they sporulate at lower temperatures down to 6ºC. Secondly they produce larger lesions with more spores - 30-40,000 spores per square cm of lesion! Thirdly they are biotrophic. That is they infect the leaf but don’t kill it. This means an even longer sporulation period with even more spores. Finally they have a shorter latent period (the time from inoculation to sporulation).”
“So we have aggressive polycyclic blight, starting earlier in colder conditions and producing more spores at a faster rate. This surge in early infection has caught some off guard in recent years. These days I advise growers to start earlier at the rosette stage and then keep spray intervals to a maximum of seven days, alternating fungicides with different strengths and modes of action. In particular anti-sporulant activity is important as well as being effective on both leaf and stem blight,” says Mr Anderson.
Eric regards Invader as a product with strong anti-sporulant ability. “It is a very useful fungicide, particularly at the higher dose rate of 2.4 kg/ha. It is effective on both stem and foliar blight and I see it as one of the better products for controlling blight spores. Together with Infinito (fluopicolide +propamocarb), I see Invader as one of the most effective anti-sporulant and sporicidal fungicides. Alternating these two fungicides during rapid canopy and into stable canopy phases would be an effective, robust programme.”
Paul Goddard points out that Invader is active against all the key stages in the blight life cycle including oospores and zoospores. “Its anti-sporulant activity reduces inoculum to prevent spread within the canopy and reduces the chances of tuber blight developing. Invader also delivers 1600 gm of mancozeb which will have a bonus effect on early blight (Alternaria), whilst it is controlling late blight at the same time. Invader has really come into its own with the high rate and activity across all parts of the life cycle.”
Eric Anderson warns growers to be wary of using Smith Periods alone to initiate spraying. “Both the 6_A1 and 13_A2 strains sporulate outside the Smith Period criteria. I think there is too much focus on temperature and not enough on Relative Humidity and leaf wetness. A more tightly defined humidity criterion would be welcomed. Common sense indicates that when it is warm and dry, there is limited spread of disease. But if it is raining or overcast, blight risk will be higher. Smith Periods are useful to give a general indication of blight, but they now underestimate true risk.”
He also says growers need to be vigilant about primary sources of blight. “Some blight is seed borne, some comes from potato volunteers in nearby crops and some from potato dumps. This year there are more potatoes dumps as a consequence of the mild autumn, very little frost and difficult marketing conditions. These dumps will give rise to blight very easily and so must be covered in black plastic sheeting with edges covered in soil. Keeping blight out of the crop initially is vitally important.”