Powdery Mildew

Key Points

• Quinoxyfen products protect all new growth
• Movement occurs in and around the plant
• Protection can last up to 8 weeks
• Plant stress relieved leading to increased yields

Introduction

Powdery mildew in cereals — Erysiphe graminis — reduces yield by draining the plant of nutrients, reducing root growth, increasing plant respiration, reducing green leaf area and blocking photosynthetic pathways. In cereals yield reductions of 40% have been recorded as directly attributable to mildew but losses of 10 – 15% are more common.

Powdery mildew affects several yield producing components. Early attacks can reduce rooting and tiller development. Attacks mid-season put the cereal plant under stress through increased respiration, diversion of nutrients, cuticular erosion and structural damage allowing other diseases to invade the plant. This results in a reduction of ear numbers, grain sites, grain size and quality. Plants that are kept free of mildew do not need to activate their genetic defence mechanisms and are therefore more efficient and productive.

Biology

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 in to 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 wheat or barley, 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.

Product Choice

Quinoxyfen is available as a single active ingredient product sold as Fortress*. It is also available in combination with the curative active ingredient fenpropimorph, sold as Orka*.

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 cereal crop canopy is minimal allowing long-term control from an appropriate rate of quinoxyfen.

Quinoxyfen is highly active against powdery mildew. Under field conditions 150g of active ingredient per hectare has been shown to give at least 42 days protection from infection. The Rate Flexibility Guide for Fortress shows the persistence in effect that can be expected.

To assist in choosing the most appropriate rate it is important to use the Cereal Powdery Mildew Risk Assessment Chart (187KB PDF) found in technical literature supporting quinoxyfen products. Depending on the outcome of the Risk Assessment, Fortress should then be applied at the most appropriate rate in 200 litres of water per hectare.

Visit the product pages for more comprehensive information about Fortress and Orka.