A changing crop mix has seen the rise of viruses and Sclerotinia in broadleaf crops
Viruses and Sclerotinia stem rot are two disease threats that have come under increased scrutiny in New South Wales in recent years. The rise in these diseases has resulted from an increasing frequency of broadleaf crops grown in continuous cropping programs. Pulse and oilseed viruses can easily fly under the radar with symptoms that are often indistinguishable from those caused by abiotic stress factors and herbicides.
A virus diagnostics laboratory was established at the Department of Primary Industries’ Tamworth Agricultural Institute with GRDC investment in 2015. The main virus diagnostic tool used is tissue-blot immunoassay, which is a quick and economic method used to process large numbers of individual plant samples.
In 2020, more than 7000 pulse samples, either submitted by growers and agronomists or collected by researchers during surveys or from on-farm field trials, were tested for up to eight viruses or virus groups (Table 1).
Table 1: The main viruses detected in collected and submitted samples of pulses grown in NSW in 2020. Detection rates greater than 10 per cent are in bold. Viruses detected in less than five plants (subclover stunt virus and clover yellow vein virus) are not included.
Crop | Number of plants tested | Percentage of positive plants as determined | ||||
|---|---|---|---|---|---|---|
Bean leaf roll virus (BLRV) | Luteoviruses | Bean yellow mosaic virus (BYMV) | Alfalfa mosaic virus (AMV) | Cucumber mosaic virus (CMV) | ||
Faba beans | 2236 | 2 | 5 | 62 | 12 | 0 |
Chickpeas | 2008 | 0 | 1 | 0 | 6 | 1 |
Lentils | 1794 | 2 | 1 | 1 | 28 | 31 |
Lupins | 576 | 1 | 1 | 13 | 17 | 59 |
Medics | 335 | 5 | 15 | 15 | 8 | 0 |
Vetches | 108 | 0 | 0 | 9 | 4 | 0 |
Lucerne | 70 | 1 | 0 | 0 | 27 | 0 |
Other pasture legume species | 68 | 0 | 2 | 0 | 0 | 0 |
Source: Joop van Leur
The results showed that a severe virus epidemic in faba bean crops in northern NSW, characterised by a wide range of symptoms, was mainly caused by bean yellow mosaic virus (BYMV) with localised co-infections by alfalfa mosaic virus (AMV). High levels of cucumber mosaic virus (CMV) were also detected in narrow-leafed lupins and lentils.
A seed testing program has been initiated for CMV in narrow-leafed lupins and BYMV in faba beans.
Results showed that in-crop CMV infection levels in narrow-leafed lupins were strongly related to seed-transmitted virus and growers should not use seed from CMV infected paddocks.
While testing showed that seed transmission of BYMV is not of importance in commercial faba bean crops, growers are advised to avoid using seed from severely-infected paddocks as seedling vigour may be reduced, making the crop more vulnerable to aphid-driven viral infection
Sclerotinia
The Sclerotinia fungus is a good example of how the increasing frequency of broadleaf crops has made it easier for the fungus to persist and damage crops when conditions are favourable.
Pulse crop surveys undertaken during the dry seasons of 2018 and 2019 across southern and central NSW detected the disease as discreet infections of the upper taproot and stem base within mainly chickpea and narrow-leafed lupin crops. These symptoms could have been easily missed or mistaken for drought stress.
With more favourable conditions in 2020, surveys detected Sclerotinia in 62 per cent of the 45 pulse crops tested, including narrow-leafed and albus lupins, field peas, chickpeas, faba beans and lentils. The highly favourable conditions in the spring of 2020 meant even low levels of fungal inoculum caused significant levels of disease.
Sclerotinia carryover from 2020 is a significant risk for crops in 2021 and beyond.
The sclerotia (survival structures of the fungus) produced in 2020 present a significant risk to broadleaf crops in 2021 and beyond. Sclerotia can readily survive in soils for at least five years – and sometimes up to 10 years. T
his presents a high disease risk for growers following double-break rotations where canola is sown after a pulse crop, especially in medium to high-rainfall districts. In these situations, growers will need to budget for foliar fungicide applications to manage Sclerotinia disease or opt for a non-host crop.
The NSW Department of Primary Industries provides diagnostics for pulse and oilseed diseases to NSW growers and advisers, as well as research into disease behaviour and management, with GRDC co-investment, through the Grains Agronomy and Pathology Partnership (GAPP).
More information: Joop van Leur, 0427 928 018, joop.vanleur@dpi.nsw.gov.au; Dr Kurt Lindbeck, 02 6938 1608, kurt.lindbeck@dpi.nsw.gov.au