Tests show that starting too late with alternaria spraying can easily cost 2.500 kilos of yield. This leads to a loss that far exceeds the costs of alternaria spraying. "Starting too late always costs money."
"It may seem a bit early to start talking about alternaria now," says Bert Westhoff at the beginning of the interview. "But you can often find the fungus in your crop six to eight weeks after emergence. And thanks to the early spring, it will soon be that way on many plots." As far as the technical product advisor of BASF is concerned, potato growers see his announcement as a well-intentioned warning. "We have seen in our trials that the correct timing of the alternaria sprays is crucial for good control. If you start too late, it always costs money. So it is better to be alert in time."
taste
The reason for the BASF trials lies in the changed package of agents in recent years. "When we were still allowed to use mancozeb, you often gave a little protection against alternaria from the start of the spraying season," says Westhoff. "As a result, you were not immediately punished if you started a little too late with the specific alternaria agents. But in the current situation, the moment at which you start with the alternaria agents determines the success of your approach."
The trials that BASF conducted in 2024 were in consumption and starch plots in Dordrecht, Emmeloord and Valthermond. Westhoff: "We tried out various resource schedules, but the most important variable was the starting moment: early, normal or late. The most important lesson from all the trials was that a late start of the alternaria spraying quickly resulted in a yield loss of 2,5 tonnes. Everyone can do their own calculations on that, but for consumption potatoes of 30 cents, that amounts to a loss of €750. While you spend around €100 on resource costs for four sprayings."
Practical advice
Summarizing all the results, Westhoff comes to the following practical advice. "Always do the first alternaria spraying in the period between the beginning and halfway through flowering; then you are usually talking about the first half of July. Then you keep the crop healthy with four to five sprayings in the period of maximum production." Westhoff also has a tip for the choice of agents. "In any case, always alternate agents. belanty and Propulse* are currently the strongest alternaria agents. From the point of view of resistance development, it is best to start with Belanty. And that agent also has the strongest side effect on black speckle and sclerotinia."
What is alternaria
Alternaria is a fungus that can occur in dozens of crops in just as many variants. In potatoes we know A. solani and A. alternata. It is a weak parasite with a preference for weakened (older) tissue, but it can also attack healthy tissue. The fungus survives with spores on dead material in the soil. The conditions that alternaria needs for sporulation and infection are very similar to those of phytophthora: free moisture or a very high RH. Sporulation occurs at temperatures between 5 and 30 degrees, the optimal germination temperature is between 20 and 30 degrees. An alternation of dry and wet periods is ideal for alternaria. Spores are spread by splashing water or wind. The visible symptoms are the sharply defined brown to brown/black leaf spots, often with a yellow edge on the upper side of the leaf. Later, the typical concentric rings are visible in these larger spots. Alternaria can already raise its head from the beginning of flowering, so in June. The plant is also more sensitive to fungi during this period.
*Propulse is a registered trademark of Bayer Crop Science.