While the Irish acreage of winter oilseed rape (OSR) is well down year-on-year, those crops that were planted out are looking tremendously well.
Given the continuing dry and mild conditions, most crops now have large canopies.
Significantly, disease-related issues are now becoming apparent. But the question is: do they constitute a yield reduction threat that requires grower intervention at this stage?
Teagasc plant pathologist, Dr. Steven Kildea, addresses this matter courtesy of his contribution to the current issue of the Tillage Edge podcast.
“We have to look back at the last couple of months and see what has happened there. A mild and, essentially, dry period has been very conducive for good crop growth.
“As a result, there are some really good oilseed rape crops out there at the present time: good canopies and tremendous yiled potential.
“The weather conditions have been absolutely ideal,” Dr. Kildea said.
Winter oilseed rape
According to the Teagasc research scientist, the one downside to the excellent growing conditions has been the enhanced risk of crop exposure to fungal disease.
“Chief among these at the present time is phoma.
“It is characterised by greyish-coloured leaf spots, which feature black pigmentation in the middle of them.
“Phoma at this time of the year is not that big of a problem for oilseed rape plants,” he added.
However, later in the spring it can spread to the stems and cause a disease called: stem canker.
“But for this to happen the fungus needs to grow from the leaf to the petiole and then into the stem.
“So, with bigger canopies it can take that little bit longer for the disease to get into the stem,” he explained.
According to Dr. Kildea, the “milder the weather the quicker will be the growth of the fungus”.
But as we are getting into a period when weather conditions get that little bit cooler, large canopies should be a direct asset to the OSR plants.
“This means that it will take that much longer for the disease to spread around the plant,” he continued.
Stem canker leads to the early senescence of OSR plants.
But as Dr. Kildea indicated, the jury is still out on whether Phoma can lead to very significant reductions in rape yields come harvest time.
He said: “Canker curtails the amounts of water and nutrients getting into the crop during the late spring, early summer period.
“And, it is in this way that the disease impacts on final rape yields. But the phoma disease must get form the leaf t the stem in order to have a yield reducing impact.
“In reality, Phoma may not be that big of an issue under Irish growing conditions. However, it is a major disease threat in the UK.”
Light leaf spot
Light leaf spot is the other disease that can impact OSR crops at this time of the year.
However, not seeing the actual presence of the disease on plants does not mean that the disease is absent within a specific crop.
Dr. Kildea explained: “This is particularly so in crops that have large leaf covers.
“It’s a case of missing the small lesions. However, in smaller crops with smaller leaves, it’s that much easier to pick up on the lesions.
“We are talking about a wet weather disease. Temperatures were quite high over recent weeks, whereas light leaf spot tends to favour cooler conditions, when it comes to its spread throughout a crop.
“Temperatures drop during the month of November. So this development, in combination with the heavy rain fall, can provide the almost perfect conditions for the growth of light leaf spot.”
Recent years have seen the development of OSR varieties with a degree of resistance to light leaf spot.
“The greater the inherent levels of light leaf spot resistance that we can bring to bear, the more impactful will be the integrated pest control management systems that we can develop.
“Once light leaf spot gets going within a crop, it will produce a lot of inoculum.
“And even in a resistant variety, come the early spring and if the conditions are right, we could have an epidemic of the disease on our hands. This is something that I would be keeping an eye on,” Dr. Kildea stressed.
Dr. Kildea also pointed out that light leaf spot is a polycyclic disease. In other words, the fungus creating the disease can complete its entire life cycle a number of times within the same growing season.
“This is why early detection of the problem is so important,” he added.
Light leaf spot, if left uncontrolled can reduce rape yields by up to 20%. Where control is concerned, the objective is to protect the early flower buds that appear from the middle of February onwards.
Light leaf spot “can flare up within a very short period of time. Within a fortnight it can be case of going from zero to almost epidemic proportions.
“And all of this driven by wet weather conditions.
“The real impact of the disease will be seen in the early spring at which stage it can cause serious damage,” Dr. Kildea said.
Growers wanting to gauge the level of light leaf spot infection within a crop of OSR can carry out what’s called the ‘hot box’ test.
This entails growers taking a few leaves from a growing crop and outing them in plastic bag. This is then placed in a hot press for a few days.
After this period of time, the bag can be opened and the leaves inspected.
If light leaf spot infection is present, the tell tale signs will be very evident on the leaves.
Prothiocanazole-based fungicides are the best treatment options for both light leaf spot and phoma.
“Application rates should be at the 75% recommended figure. It really is a case of getting to grips with both diseases if they have reached a threshold level.
“From a phoma perspective, it’s important to catch the disease before it spreads out from the infected leaves.
“Where light leaf spot is concerned, it will not be possible to kill-off the disease completely at this time of the year”, the plant pathologist explained.
“This is because what can be seen with the naked eye is only the tip of the iceberg with regard to the levels of disease actually present.”
It is still quite a period of time between now and the beginning of February, so:
“The aim of a light leaf spot control programme at this stage, is to ensure that fungicides applied at the flower bud emerging stage next spring will have their optimal effect.
“The last thing growers need, is to have epidemic levels of light leaf spot harbouring within crops of oilseed rape over the winter months,” he stressed.