Sclerotinia infection in oilseed rape is predominantly driven by inoculum levels and weather events at earlyto mid-flowering, but a notable decline in incidences of the disease has been seen in recent years.
Falling rates of sclerotinia are thought to be down to a fall in cycling inoculum as a result of the reduction in oilseed rape area and farmers being more on top of the disease over the last 10 years.
However, changes in weather patterns are a dominant factor, with rain events during flowering tending to be less prolonged in the last few years, says Dr Caroline Young, research plant pathologist at ADAS.
“When we analyse weather data, we used to see six to eight occasions during flower when we thought disease threat was high, but in recent years it has been more usual to have only two to three. This is because we have seen an increase in drier phases during flowering, which is when sclerotinia infection takes place.”
There are more than 400 plant species that sclerotinia can infect, from buttercups to lettuces and potatoes, making it a much broader disease than many of the fungal pathogens we are used to. Sclerotia, the resting bodies of the pathogen, can last in the soil for five to 10 years, so extended rotations will also help to reduce the possibility of infection, Dr Young says.
Sclerotinia requires at least 24 hours of continuous high humidity and night-time temperatures above 7degC for infection to occur.
“In recent years we’ve had weather that has been drier and humidity has gone down.
We’ve also had unpredictable weather where we’ve had cold phases too. If it’s not humid enough or you don’t get a continuous mild period, you don’t get infection,” says Dr Young.
Wet weather during flowering creates the biggest risk to the crop because this encourages fallen petals to stick to the crop’s leaves.
These petals act as a nutrient source for the pathogen, which then grows down the leaf stem to the main plant stem where it presents as large white lesions some two weeks after the plant has become infected.
Dr Young says: “When petals start to fall off in moist situations they will stick to the leaves of the crop and sclerotinia spores germinate on the dead petal material. The fungus infects the leaf and the fungus will grow down into the main stem.”
Early flower infections generally lead to stem lesions lower on the plant which cuts off translocation, killing the whole plant and leading to significant yield losses.
"Infections during late flowering won’t be so dramatic as those are often on the side branches of the plant"
Dr Caroline Young
“Infections during late flowering won’t be so dramatic because those infections are often on the side branches of the plant, so you’re affecting a branch rather than the whole plant,” Dr Young adds.
For growers in areas which have a high-risk for sclerotinia, Dr Young recommends a 75% to full dose protectant foliar fungicide ahead of the main petal fall phase.
She says: “There may be some crossover during flowering where light leaf spot control is still needed. However, the crop may have had several azole treatments already and in terms of resistance management for light leaf spot, it is really important not to keep putting azoles on.”
In high pressure years, yield responses to fungicides have been up to two tonnes per hectare, Dr Young says, but even in years where there is no perceived disease threat, crops responded to Filan with a 0.2t/ha uplift.
In an effort to determine the reason for this yield uplift in low disease situations, ADAS field trials over three seasons found yield is strongly related to the amount and duration of green leaf from mid-flower to seed fill.
This is because seed fill relies on current photosynthesis and unlike wheat plants, where some remobilisation of stem carbohydrates go into the seed, for OSR there is very little remobilisation of carbohydrates in the plant.
This means the longer the leaves are green and can photosynthesise, the more energy is put into filling the pods. In the trials, ADAS assessed green leaf area duration (GLAD) units to define what a ‘greener crop’ is and its impact on yield.
In untreated crops GLAD units totalled 99, compared to plots treated with Filan (boscalid) which senesced later and had 146 GLAD units, which translated to the 0.2t/ha yield uplift.
Similar trials were repeated using the fungicide Pictor (boscalid + dimoxystrobin), which gave a yield uplift of 0.25t/ha.
Clare Tucker, agronomy manager at BASF, says: “These fungicides reduce the stress hormone, ethylene in the plant and therefore keep leaves green.
“Leaves are the most important photosynthetic organ in the plant so if we can keep them going, it makes a big difference to seed fill and seed size.”
However, not all crops will respond to an increase in GLAD units if the crop already has the optimum green leaf area for its yield potential and conditions are ideal for seed-fill, she says.
“However, since we can’t predict the future weather at flowering time when fungicide decisions are made, it’s good to have some extra GLAD units in the bag.
“Filan and Pictor help keep the crop green to buffer against challenging conditions, such as high summer temperatures, dry conditions, or a dull summer where photosynthesis is poor.
“It’s an added insurance in addition to their main function of protecting against sclerotinia and any late light leaf spot spores in the pod canopy.”
"If we can keep leaves going, it makes a big difference to seed fill and seed size"
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