What’s your problem? Identifying diseases correctly
Over the last few weeks, NIAB have been running some in-field disease assessment training days for trial operators. I was lucky enough to attend one of these, and it was extremely useful.
We were looking mainly at wheat yellow rust and septoria, two diseases which can be easily mixed up when there are no yellow rust pustules or septoria picnidia present.
Although septoria does tend to leave a necrotic area while yellow rust usually leaves a chlorotic area, we learnt that the thing to do is to only record the disease if you can see the pustules or picnidia. If you can just see necrotic or chlorotic area, a green leaf area assessment should be done instead of a disease assessment. The damaged area could be due to an environmental stress, the early stage of the disease, or yellow rust which hasn’t completed its lifecycle (and therefore shouldn’t be scored) – it is impossible to know.
The exception to this is if you’ve been visiting the trial regularly in the days and weeks before the assessment, and you have previously seen the pustules/picnidia, so you can be confident which disease you are seeing. If there are enough pustules/picnidia on the surrounding necrotic/chlorotic area that you can be confident that the remaining area is attributable to disease, then you can also record the area as that disease. Remember that yellow rust pustules can wash off after heavy rain so you’ll need to turn the leaf over and look for pustules on the underside of the leaf to check in this situation.
In the advanced stages of yellow rust, black telia form, and these shouldn’t be confused with septoria picnidia.
Another difference between septoria and yellow rust is that septoria tends to constrict the leaf as it spreads across its width, whereas yellow rust doesn’t (but yellow rust may cause curling of the leaf).
Something else I learnt was that it is important not to overestimate the % disease on a leaf or plot. It can often look worse than it is so always think twice when scoring a plot. The following is a guideline for disease levels for each score:
|% Infection||Yellow rust||Septoria|
|0||No infection observed||No infection observed|
|0.1||1 stripes per tiller||1 lesion per 10 tiller|
|1||2 stripes per leaf||2 small lesions per tiller|
|5||Most tillers infected but some top leaves uninfected||Small lesions beginning to form areas of dead tissue across width of leaf|
|10||All leaves infected but leaves appear green overall||2 lower leaves – large
areas of diseased tissue
some covering 1/3 of leaf
|25||Leaves appear ½ infected ½ green|
|50||Leaves appear more infected than green|
|75||Very little green leaf tissue left|
|100||Leaves dead – no green tissue left|
And here are some examples of % yellow rust:
And finally, if you find a moment, the UKCPVS are looking for fresh samples of wheat yellow rust, brown rust, and mildew, and of barley powdery mildew from across the country so please send some in! The sampling instructions are available here: http://www.niab.com/uploads/files/ukcpvs_sampling_note.pdf
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