Canola growers are asked to carefully weigh up the economics of spraying fungicide on canola this year, with the dry start in some areas of the southern region contributing to low levels of blackleg early in the season.
In other areas where the start to the season has been more favourable, white leaf spot is showing up in some canola crops.
Downy mildew is also starting to appear in some canola crops and, if conditions are conducive later in the season, sclerotinia stem rot could make an appearance in spring during flowering.
Marcroft Grains Pathology principal Dr Steve Marcroft says it is difficult for growers to develop a holistic strategy that considers all canola diseases. However, in considering fungicide control, he says it is important growers weigh up variety choice, seasonal conditions and level of disease risk and seed or in-furrow applied fungicides.
Blackleg was a significant issue for canola growers in 2016, largely thanks to the above average season. The disease thrives under continual wet conditions for spore release and infection.
However, seasonal conditions thus far in 2017 have not been conducive for blackleg in most areas of the southern region.
Dr Marcroft says fungicide application was recommended in the lead-up to the 2017 season to protect moderately susceptible (MS) cultivars, particularly those sown in the medium and high-rainfall zones of the southern region where blackleg risk was higher.
“If a canola crop is already past the vulnerable seedling stage (one to four leaf stage), and has no or few leaf lesions, it is likely that plant won’t develop severe crown canker and may not benefit from a foliar fungicide application,” he says.
“If the crop was sown later, has a MS or lower blackleg rating and still in the vulnerable seedling stage in mid-June it may develop severe crown canker and therefore benefit from a foliar fungicide application.
“Growers should monitor their canola crops for blackleg lesions on the first four leaves, estimate the potential crop yield and decide if it is economical to protect the crop.”
Dr Marcroft says foliar fungicide has highest efficacy against blackleg crown canker if applied at the 4-6 leaf stage, but is still very effective up to the 8-9 leaf stage.
“If growers are unsure about the blackleg severity on their crop and the potential yield, they can wait until the 8-9 leaf growth stage before making their decision,” he says.
Table 1: Examples of the severity of blackleg infection and suggested management. Source: Marcroft, 2017.
|Example 1: Very severe blackleg leaf lesions in 2016. Growers are likely to get return from applying foliar fungicide even on moderately resistant (MR) cultivars.||Example 2: High blackleg leaf lesions in 2016. Growers may get a return from applying foliar fungicide on MR cultivars. Foliar fungicide is recommended on MS cultivars.||Example 3: Normal blackleg severity. Growers are unlikely to get return from applying fungicide on MR cultivars but may get return on MS cultivars. Growers should make a decision at 8-9 leaf stage whether to apply foliar fungicide.|
Because of the favourable conditions and therefore high yields in 2016, many growers had great success and good economic return from applying a fungicide spray to control blackleg.
“Even if growers have budgeted a fungicide application, they should still assess disease risk and seasonal conditions before applying so they can judge whether or not they will get an economic return from applying fungicide this year,” Dr Marcroft says.
“With the way the season has turned out so far, most growers should be able to reassess and save some money by not applying fungicide.
“In regard to MS varieties, those sown later which still haven’t reached the eight-leaf stage may need a fungicide application if blackleg presents itself.”
Dr Marcroft says it is too early to tell whether what effect the low levels of blackleg will have on the severity of upper canopy infection later in the season.
Upper canopy infection of blackleg causes lesions on the stems, branches, flowers and pods of canola plants. It has become more severe in recent years with the shift towards earlier sowing of canola.
“In the past, canola crops were generally vegetative in May, June and July before flowering in August,” Dr Marcroft says.
“However, in recent years more growers are sowing earlier, which means crops are elongating in June or July and, instead of causing lesions on the leaves during the vegetative stage, blackleg is also causing lesions on those upper parts of the plant.”
Dr Marcroft says the early low levels of blackleg may not make any difference to the situation later in the year. Rather, it will now depend on rainfall during July.
“If we get wet, showery weather later when the crops are flowering in July then it is likely we will see some incidence of upper canopy infection,” Dr Marcroft says.
“The situation could really change quickly depending on what the weather does.”
White leaf spot
There have been increasing cases of white leaf spot in canola crops early in the 2017 growing season, Dr Marcroft says. However, white leaf spot is usually not severe enough in Australia to cause yield loss.
However, he says if white leaf is severe enough it can result in significant defoliation, thereby reducing plant vigour and causing yield loss.
“White leaf spot is very common but normally only occurs on the oldest leaves of the plant, near the soil surface where constant wetness occurs,” Dr Marcroft says.
“If conditions are conducive it can move up the canopy, infecting younger leaves reducing the leaf area significantly. This is when yield loss may occur.”
White leaf spot lesions can occur on leaves and are a greyish white to light brown colour. The lesions can grow up to a centimetre in diameter and merge with other white leaf spot lesions to form larger, irregular shaped lesions.
Lesions associated with white leaf spot do not contain pycnidia, the black dots that are a characteristic of blackleg.
“The issue with white leaf spot is that because it can defoliate the plant and reduce green leaf area, it can reduce the plant’s early vigour,” Dr Marcroft says.
White leaf spot is caused by the fungus Mycosphaerella capsellae and survives during the fallow period on canola stubble as thick-walled mycelium.
“Under favourable autumn and winter conditions, that mycelium produces ascospores which cause leaf lesions,” Dr Marcroft says.
“These initial lesions go on to produce new wind-borne spores that cause the rapid spread of disease throughout the crop. The disease is not usually seed-borne, but it can be spread by infected seeds or infected debris with the seed.”
There are currently no registered fungicides available in Australia for the control of white leaf spot in canola.
Instead, Dr Marcroft advises an integrated approach to managing white leaf spot which includes controlling volunteer canola and cruciferous weeds such as wild radish. Diverse crop rotations and sowing away from the previous year’s canola stubble can also help to reduce infection of white leaf spot from wind-borne spores.
Much like white leaf spot, downy mildew also attacks the older leaves of canola plants and also can caused reduced leaf area and vigour in canola crops.
Dr Marcroft says that, typically, younger leaves remain unaffected and ultimately the disease has little bearing on the crop yield.
In regard to sclerotinia, there are three key risks to disease development. These include:
- Spring rainfall: Epidemics of sclerotinia stem rot generally occur in regions with reliable spring rainfall and long flowering periods for canola.
- Frequency of outbreaks: A history of frequent sclerotinia stem rot outbreaks in a region is a good guide to the risk of an outbreak. Paddocks with a recent history of sclerotinia are a good indicator of potential risk, the risk is also increased for adjacent paddocks. It is also important to consider the frequency of canola, as the crop is a very good host of the disease and it can quickly build up levels of soil-borne sclerotia.
- Start of flowering: The start of flowering can also determine the severity of a sclerotinia outbreak. Spore release, petal infection and stem infection have an increased chance of occurring when conditions are wet for longer than 48 hours. Canola crops that flower earlier in winter, when conditions are cooler and wetter, are more prone to disease development.
Growers in areas at high risk of sclerotinia later this season should consider the use of a foliar fungicide between the 20 per cent and 30 per cent bloom stage which can reduce infection levels. Most registered products can be applied up to the 50 per cent bloom stage.