Downy mildew of potato

When to look

• The disease appears in the second half of summer, in the period from flowering to ripening of the potatoes. In damp places in lowlands and floodplains it appears earlier.
• Significant spread and infection of plants were recorded mainly in the second half of the growing season. The highest level of infection is in the period between flowering and plant maturity.
• At the budding stage, crops are surveyed daily: first early-maturing varieties, then mid- and late-maturing ones.
• Disease spread is recorded from the moment the disease appears. The next assessment is made ten days later, and after that by potato growth stages. Mandatory assessments are carried out at budding, at flowering and at the start of ripening (when the lower leaves die off).

How to count

• Potato crops from stores where late blight rot was found on the tubers are surveyed first.
• Fields in low-lying areas, fields with excessive nitrogen fertiliser applications and fields on peat soils need constant monitoring for the disease.
• To spot the first signs, the haulm is parted and the lower leaves lying on or close to the soil are examined carefully. If there are late blight spots on the leaves or sprouts with rot, the plant is dug around and the mother tuber is examined.
• Once primary disease foci are found, neighbouring potato fields up to 500 m away are carefully inspected. Start with the nearest field and take the direction of the prevailing winds into account.
• Haulm infection is assessed by examining plants evenly along the two diagonals of the plot.
• On a field of up to 50 ha, 20 samples are taken. If the field is larger than 50 ha, two more samples are added for every further 10 ha. In each sample, five plants along the row are assessed.
• The degree of infection of each plant is estimated by eye on a six-point scale based on the share of leaf surface affected (0 points: no infection; 1 point: up to 10 % of the leaf surface affected).

No rate and no spray timing here: both come from the label and the register.

Agronomy

According to the Institute of Potato Growing, the following varieties have relative field resistance to the disease: Ariia, Vzirets, Myroslava, Hurman, Kniahynia, Radomysl, Lietana, Vyhoda, Okolytsia, Yavir, Slovianka, Sluch, Chervona Ruta, Feia, Poliske Dzherelo, Bazaliia, Slauta, Strumok, Predslava, Synhaivka, Solokha, Tradytsiia, Fotyniia, Veresivka, Oleksandryt, Bazhana, Zhytnytsia, Alians, Mezhyrichka 11, Rodynna, Avanhard, Spokusa, Medeia, Misteriia, Sontsedar.

Favourable conditions

• The disease appears earlier in damp places: in lowlands, floodplains, near rivers, lakes and other water bodies, among mountains and shelterbelts, where dense fogs often settle and heavy dews fall.
• Early-maturing susceptible varieties and dense stands are infected first, where the haulm is shaded and the air humidity is raised.

Every fact rests on a quote from its source, and each quote was checked against the page it is printed on.

1. EPPO Global Database · European and Mediterranean Plant Protection Organization
2. Станкевич С. В., Положенець В. М., Немерицька Л. В., Журавська І. А., «Моніторинг хвороб сільськогосподарських культур», навчальний посібник, 301 с. · Інститут біоенергетичних культур і цукрових буряків НААН, Державний біотехнологічний університет; Житомир: Рута, 2022 · p. 214, 217, 218
3. Прогноз фітосанітарного стану агроценозів України та рекомендації щодо захисту рослин у 2025 році · Держпродспоживслужба, НААН, НАН · p. 152, 153
4. Photos: Sigurd Anders Svalestad, GBIF, CC BY 4.0 · kitbeard, iNaturalist, CC BY 4.0