Powdery mildew in sugar beet — Erysiphe betae — reduces yield by draining the plant of nutrients, reducing root growth, increasing plant respiration, reducing green leaf area and blocking photosynthetic pathways. In sugar beet crops, yield reduction and suppression of sugar content can occur as a consequence of powdery mildew infection. The effect is greatest in late-drilled crops. Plants that are kept free of mildew do not need to activate their genetic defence mechanisms and are therefore more efficient and productive.


Like all fungi, powdery mildew has a defined life cycle, producing millions of airborne spores throughout its course. In favourable conditions, spores germinate on the leaf surfaces forming structures called appressoria. Germination occurs between 2°C and 30°C with an optimum range of 15°C to 20°C. The appressorium forms an infection peg and then hyphae penetrate the plant tissue, invading cells and parasitising the plant. A feeding organ, haustorium, is formed.

Sustained by the plant the fungus is able to develop and grow, forming conidia which in turn produce conidiospores. Growth can occur from slightly negative temperatures up to 30°C, the optimum range again being 15°C to 20°C. Conidiospores then break off and are dispersed by the wind ready to produce a new infection.

Sexual reproduction occurs when conditions are unfavourable for conidia production. The hyphae of different fungal strains fuse on the surface of plant tissue to produce a sexual structure which grows into a resting spore. This spore contains asci which in turn contain ascospores. When conditions become favourable, these ascospores infect growing plants in the same way as conidiospores. These two methods of reproduction mean that host plants, such as sugar beet, are under constant threat from mildew.

When conditions are favourable epidemics can occur as powdery mildew can progress through its life cycle stages in just three to four days (20°C). Under less favourable conditions this latent period, the time between infection and the development of visible symptoms, can take longer e.g. 12 days at 10°C and 30 days at -2°C.

In the UK sugar beet crop, the extent of the powdery mildew epidemic each year is dependent on the number of frosts in February and March. Sugar beet powdery mildew overwinters as haustoria or mycelia on groundkeeper crowns, wild beet or beet volunteers. A higher number of frosts reduce the amount of inoculum available to infect newly drilled crops. Each year a forecast is issued of the likely severity of powdery mildew in sugar beet.

Product Choice

Quinoxyfen is available as a single active ingredient product sold as Fortress*.

Quinoxyfen’s primary activity on the mildew pathogen is the prevention of the formation of appressoria. Secondary activity affects spore germination and spore viability. Instead of forming appressoria spores landing on treated leaf surfaces produce a germ tube that elongates until its internal reserves are exhausted. Once starved of resource the fungus dies.

After application movement of quinoxyfen occurs both in and around the plant protecting new and existing growth. Quinoxyfen quickly binds to the plant surface, providing rainfastness within 1 hour, and then begins a slow but constant movement around the plant. Primary redistribution is by local vapour movement close to the leaf boundary layer, ensuring complete protection of new and existing growth after application. Entry into the plant by penetration through the outer waxy layer allows movement through the plant’s translocation system. Systemic activity is slow but can occur both via the phloem and xylem.

In this way quinoxyfen distributes itself evenly all around the plant. After application product distribution in cereal plants reaches equilibrium of 16% on the leaf surface, 18% in leaf wax with 66% of the product remaining within the plant tissue. The slow, constant migration of quinoxyfen is maintained over time by drawing on the reserves of active ingredient held within the component parts of the plant and its leaf surface. Air movement within the crop canopy has an influence on the longevity of control achieved. Air movement in a mature sugar beet crop canopy is minimal allowing long-term control from an appropriate rate of quinoxyfen.

Quinoxyfen is highly active against powdery mildew in sugar beet. The Rate Flexibility Guide for Fortress shows the length of protection that can be expected in sugar beet.

Disease Pressure
Rate of Fortress Applied
(>25% infection)
4 – 5 weeks
8 – 9 weeks
8 – 9 weeks
>8 – 9 weeks
(5 – 25% infection)
4 – 7 weeks
9 – 13 weeks
9 – 13 weeks
>9 – 13 weeks


To assist in choosing the most suitable rate, or whether a sequence of two applications is more appropriate, it is important to consider how favourable conditions are for continuing disease pressure and the likely lifting date of the crop to be treated. A single application of up to 300 ml/ha of Fortress can be made, or a sequence of up to 400 ml/ha (2 x 200 ml/ha). The sequence approach may be beneficial where high and sustained levels of powdery mildew are expected, where re-infestation is likely, or where a later harvest is planned. Fortress should then be applied at the most appropriate rate in 200 litres of water per hectare. Applications should begin at the first sign of disease and, if a sequence is being used, a further application can be made up to 28 days before