OIL SEED RAPE CANOPY MANAGEMENT

Written by Jayne Smyth

Green Area Index is the ratio of green leaf and stem area to the area of ground on which the crop is growing. GAI will vary from crop to crop (and within crops) depending on sowing date, available nitrogen and grazing by pigeons. Assessment of the Green Area Index will guide when and how nitrogen should be applied.

The following points should be considered: • A target canopy cover of GAI 3.5 at the start of flowering to deliver maximum yield this should equate to between 6,000 and 8,000 pods per square metre. • Nitrogen has a large influence on the canopy size. • It requires 175 kg/ha N (from a combination of soil N release, canopy accumulations and applied nitrogen) to achieve GAI 3.5 by flowering. • Start nitrogen (and sulphur) application on thin/grazed crops as soon as growth commences in spring. • Hold back 35-60 kg N/ha in all crops to be spread as near as practically possible to mid-flowering. This is to prolong green-leaf area during pod-fill. Leaves are the most efficient part of the rape plant to convert sunlight into yield. • For each 0.5 t/ha of yield above 3.5 t/ha up to a maximum of 5 t/ha the crops needs 30 kg/ha of Nitrogen. • Fungicides with growth regulation properties can be used to manipulate crop growth habit (and control disease) but check GAI before application.

GAI = Ground Area Index and is a measure of how much green plant material is in a crop. It can be measured by weight (best suited to wide row spacing’s*) or by digital photograph (best suited to narrow row spacing’s). Download the OSR GAI app from the app store onto your smart phone and it will calculate the GAI from the photograph.

If you don’t have a smart phone you can still measure the GAI by following the steps below. *Measuring GAI with wide row spacing’s. 1. Cut all plants from a meter row. 2. Weigh the fresh weight collected in kilos. 3. Divide the weights by 0.61 (for 24 inch spaced crop) to take into account the row spacing. 4. Multiply the final figure by 0.8 to get the GAI of the OSR crop.

Sclerotinia in Oil Seed Rape

Sclerotinia stem rot is usually the main disease to consider during the flowering stages of oilseed rape. Although the infection cycle of Sclerotinia sclerotiorum is complex, a good understanding of the three main risk factors – the presence of sclerotinia inoculum, warm and humid weather conditions and crops in flower – will help you target control.

Symptoms

The pathogen that causes sclerotinia (Sclerotinia sclerotiorum) in oilseed rape can also infect other crops, such as potatoes, peas, carrots and many other vegetables. In oilseed rape, when lower main stems are infected, white stem lesions form with black sclerotia within the stem cavity. Infection can cut off the supply of food and water to the canopy. In extreme situations, whole plants can be lost. If infection occurs during later flowering, infection tends to be limited to the lateral branches with a much less damaging effect on yield.

Life cycle

Sclerotinia in oilseed rape is caused by the fungal pathogen Sclerotinia sclerotiorum. The infection cycle is complex and requires three main factors: the presence of sclerotinia inoculum, warm and humid weather conditions and crops in flower.

In spring, sclerotinia fungal resting bodies (sclerotia) in the soil germinate. This usually occurs when soils are moist and warm (>10°C). Pale brown fruiting bodies (apothecia) are then produced that release airborne ascospores.

These spores land on oilseed rape petals and stick to other plant surfaces (especially if there is light rain). The spores require external nutrients, derived from petals and pollen, to enable them to germinate and infect the plant. Plant infection also requires long periods of high humidity and night-time temperatures above 7°C. Sclerotinia spreads from leaf lesions to the stem. Stem lesions increase in size, girdling the stem resulting in premature ripening and weakening stems. Eventually, stems can lodge. New sclerotia are formed within the stem cavity and roots. These are often returned to soil at harvest.

Importance

Soil-borne diseases, such as sclerotinia stem rot, clubroot and verticillium wilt (also known as verticillium stem stripe), have increased in recent years, favoured by shorter rotations. Sclerotinia stem rot is usually the main disease to consider at flowering. The disease often causes little damage but can halve yields when severe. It also poses a threat to other broad-leaved crops in the rotation.

Risk factors

• Rotations (short rotations, especially those that include other susceptible crop hosts increase the risk) • A history of previous sclerotinia infection on-farm • Favourable weather (e.g. mild day and night temperatures with light rain or dew) • Crop growth stage (crop in flower) • Presence of sclerotinia inoculum • Germination activity of sclerotia • Fungicide choice and timing (protectant fungicides need to be applied before infection to achieve good control) Management

It may be necessary to extend rotations at sites with severe epidemics.

Fungicide timing is important for good control, as products available to control sclerotinia stem rot are protectants and have little or no curative activity.

The optimum timing for a single spray is, usually, just before mid-flowering on the main raceme and prior to significant petal fall. Treatments provide good control for about three weeks. Two sprays may, therefore, be required to protect crops at high risk sites throughout the flowering period (especially when the flowering period is extended).

Fungicides differ in their physiological effects on the crop and also efficacy against other diseases that may require control during flowering (e.g. light leaf spot). Whether additional disease control or growth regulation (e.g. to decrease lodging risk) is required should be considered when selecting products.

Strategies are required to minimise the risks of selecting for fungicide resistant strains of sclerotinia and other pathogens. No resistance to fungicides has been reported for sclerotinia and likelihood of it occurring can be decreased by using mixtures, co-formulated products and products with a different mode of action throughout flowering and the whole fungicide programme. Strains with decreased sensitivity to SDHIs and MBCs have been reported in France demonstrating the importance of implementing strategies now. It is important that fungicide resistance management guidelines are followed (e.g. do not rely on a single mode of action for sclerotinia stem rot control).