End of season round up – Disease
7 Aug 2018
As year 2 of our Real Results OSR site monitoring and in-season updates comes to a close, we look back at the season with our ADAS experts. Here, Caroline Young, Philip Walker and Tim Boor reflect on the incidence of sclerotinia, light leaf spot and phoma this past season.
Sclerotinia (Caroline Young)
In general, sclerotinia infection has been lower than expected, as judged from infection risk assessed during early and mid-flower. For many sites, there were phases of weather during flowering that were considered high risk for infection, namely temperatures above 7oC and humidity over 80%, lasting for longer than the threshold duration of 23 hours.
“Humidity declined with the onset of the dry weather, and as crops moved into late flower, the sclerotinia infection risk tended to decrease”
These occurred mainly in the first two weeks of April and at the end of April. After that, humidity declined with the onset of the dry weather, and as crops moved into late flower, the sclerotinia infection risk tended to decrease.
One thing to note for 2018 that was different to past years, was the more frequent occurrence of high risk weather phases in the east, and in coastal areas south and north, associated with reports of mist. Petal tests indicated high levels of inoculum at many sites, at the same time as high-risk weather, e.g. Devon, Hereford, Norfolk and Yorkshire sites all had 75% petals testing positive at early and/or mid-flower, and the Kent site had 100% positive at these times. The Scottish site petals were low at first, but 50% positive by late flower.
Although conducive weather and high inoculum appeared to coincide at many sites, infection by sclerotinia was low for many trials, as assessed from % incidence of infected plants in untreated trial plots. All trials were sited in fields in high-risk areas.
Light leaf spot (Philip Walker)
Light leaf spot developed later than in previous seasons, with minimal symptoms reported in the autumn, and first symptoms reported in the majority of crops in early February.
“Heavy rainfall in early April saw the disease advance rapidly”
Heavy rainfall in early April saw the disease advance rapidly at the Herefordshire and North Yorkshire sites, with 100% of plants affected in susceptible varieties like Windozz (RL LLS resistance rating = 5) and 60 – 80% of plants were affected in more resistant varieties like Elgar (RL LLS rating 7).
This contrasts with the previous season, where dry spring weather generally slowed the progression of the disease. Where fungicides had been applied on resistant variety Elgar, this reduced the light leaf spot incidence to 20 – 40% of plants affected. The worst symptoms were reported in the west and north, in Herefordshire and North Yorkshire respectively, but where there had been drier weather, very minimal light leaf spot was reported in the east in Cambridgeshire.
“A clear difference was seen on varieties for stem light leaf spot severity”
In Herefordshire, a clear difference was seen on varieties for stem light leaf spot severity, with 26% seen on susceptible variety Windozz and 7.5% seen on resistant variety Elgar, showing there is a benefit in reducing infection through varietal choice. Where fungicides had been applied, a small reduction of 1 to 3% was seen on stem light leaf spot severity and 1 to 2% reduction on pod light leaf spot severity. The rapid development of light leaf spot in the spring may have limited the effectiveness of the fungicide applications. Based on the 60 to 100% of plants affected at stem extension in the spring, yield losses could be as high as 30%.
Phoma (Tim Boor)
The 2017/18 season was a stern reminder that phoma can pose a significant threat to crops. The epidemic was earlier than we would usually expect, with rainfall during August and September inducing spore release from the previous season’s infected stubble, with infections observed in many crops by early/mid-October, requiring a fungicide treatment.
Reinfection occurred around six weeks later, with a requirement for a follow up spray, particularly on susceptible varieties. The impact from phoma is different to other foliar diseases. The yield reduction occurs not from the reduction in green leaf area caused by disease symptoms, but from the infection travelling from the leaf lesion, along the petiole and into the stem, causing a canker in the stem base, which restricts the plant’s ability to uptake nutrients and water. Severe cankers can also weaken the stem base and contribute to lodging.
Clear treatment differences are apparent in canker severity between plots having received a strong fungicide programme, compared to those which did not.
“Significant yield reductions from this disease can occur”
Significant yield reductions from this disease can occur, with ADAS phoma trials in south Lincolnshire, on a highly susceptible untreated variety, found to have yield reductions of approximately 2 t/ha, compared to co-located resistant, fungicide treated varieties.
Looking ahead to next season, there will be plenty of infected stubble from the 2018 harvest. However, it is important to monitor the number of days with rain from the 1st August to determine when spore release is likely to occur, with over 20 days with rain optimal for ascospore release.
If the current weather continues, then limited spore release from crop debris is likely to delay the phoma epidemic for autumn 2018.