Phytophthora strains increasingly aggressive

The potato blight Phytophthora infestans is changing. New strains have emerged that are more aggressive than we were used to before. The new strains we mainly deal with are EU-36 and EU-37. In the Netherlands we mainly deal with EU-36. These new strains form more spots and spores. In addition, the spots grow faster.

The time between foliar infection and the onset of visible sporulating infections is 36 to 1 days shorter in this EU-2 strain than in older strains. This means that in hot and humid weather, the interval between sprays should be shortened.

Development of Phytophthora population in the Netherlands
The figure below shows a clear increase in the more aggressive strain EU-36 in the Netherlands in recent years. In 2019, this concerned 67% of the tested phytophthora populations (N=141).

Explanation of figure, data 2019

• Pink (EU-36) – 67%
• Gray (other genotypes) – 21%
• Blue (EU-13) – 7%
• Green (EU-37) – 4%
• Brown (EU-1) – 1%

Research into the effectiveness of phytophthora agents
In 2019, Wageningen UR conducted a study into the effectiveness of various agents against phytophthora. This test field was located on the location Valthermond (Dr.).

The test field was irrigated to create the right conditions for phytophthora. The fields were infected with the more aggressive Phytophthora strains. The disease pressure was therefore artificially increased in order to be able to compare the drugs with each other in terms of strength.

The chart below shows the percentage of the plant that is affected by Phytophthora. This is a comparison between Zorvec NZeb (Zorvec + Mancozeb), Zorvec NTec (Zorvec + Gachinko), agent R. (mandipropamid) and agent I. (fluopicolide + propamocarb).

Middel R. and Middel I. were sprayed every 7 days and the Zorvec NZeb and NTec every 10 days.

The results show that the product Zorvec gave better protection against phytophthora even with a longer spraying interval.. At normal pressure Zorvec protects for 10-11 days (versus 7 days when using a standard product). If the pressure is high and a lot of new leaves are added, you can go back to 4 (-5) days with the standard means. With Zorvec, the grower can then stay on a 7 (-8) day schedule.

Above you see photos taken at the trial field in Valthermond. We do not see any active sporulating tissue in the fields with Zorvec. So no white border with fresh mold fluff around the stain. This was the case with the other resources. The fungus had made an attempt to infect the leaf at Zorvec, but without success. Zorvec thus gives a stronger protection despite being applied with a three day longer spraying interval.

More information about Zorvec?
Allard Jukema – Product Manager Corteva
Email: allard.jukema@corteva.com
Phone: 06 – 2007 8675
Website: www.zorvec.nl