Cucumber Mosaic Virus – sowing healthy seed is critical

Aphid-borne virus epidemics are favoured when a significant source of virus infection is present, and climatic conditions (warm weather, rainfall sufficient for green bridge development) favour aphid buildup. Best management for seed-borne pulse viruses, such as Cucumber Mosaic Virus (CMV) starts with maintaining a healthy seed source. Sowing healthy seed removes one of the principal means by which the virus gets introduced to crops. Aphids then can’t acquire and spread the virus multiple times throughout the growing season.

Host range and symptoms of Cucumber Mosaic Virus

Cucumber mosaic virus symptoms in lentils. Source: Aftab Mohammad, Agriculture Victoria

Cucumber mosaic virus symptoms in lentils. Source: Aftab Mohammad, Agriculture Victoria

CMV’s host range is very broad including the pulses – lentils, chickpeas, narrow-leafed and yellow lupins, faba beans and field peas, and many other crops. Plants infected with CMV display a diverse array of foliage symptoms including leaf size reduction, deformation, down curling, mosaic `patterns, yellowing and reddening, and plants may be stunted. When they become infected early, as with seed-borne infection or early aphid transmission, they are often severely stunted becoming overgrown by other plants.

CMV management starts with testing

Testing representative seed samples for CMV infection before sowing and then discarding infected seed is considered best management practice so that healthy seed stocks are sown. Testing conducted by commercial seed testing labs ideally employs a 1000 seed representative sample seedling grow out and testing the seedlings for virus presence. This is particularly important for narrow-leafed and yellow lupins and lentils. CMV seed infection levels tend to be much lower in chickpeas, field peas and faba beans so they suffer less from CMV diseases. White lupins are immune to CMV.

In addition to sowing healthy seed stocks cultural management strategies applied at planting time are useful. They include sowing seed at high seeding rates with narrow row spacing and retaining stubble on the soil surface. This reduces aphid landing rates during the critical early growth phase of the crop as aphids are attracted to plants that are surrounded by visible bare earth, but repelled by ground cover and closed canopies. Weed control is also important as some weed species can harbour CMV. These control measures limit spread of the virus from nearby infected crops by aphids or internal seed-borne virus infection sources. In-crop management of CMV by insecticides that kills aphids is ineffective and not recommended. Neither is the use of aphid-resistant varieties as these too are ineffective.

Non-persistent viruses – Implications for management

CMV is a non-persistently aphid transmitted virus. This means it can be picked up by a feeding aphid vector very rapidly – within one or two seconds of probing an infected plant – then transmitted within the same very short time period while probing the next couple of healthy plants before its stylets are cleaned up such that it no longer transmits virus. The aphid then has to feed on another infected plant before it can transmit virus again. This means that while CMV can be quickly transferred between infected and healthy plants within a crop the virus doesn’t remain with the aphid for long.

Such non-persistent transmission has major implications for management as insecticides are ineffective. This is because they don’t act fast enough to kill a virus carrying aphid before it has time to visit and infect healthy plants. Also, non-colonising aphid species spread the virus so colonising aphid infestation is not necessary for spread to occur. Therefore, management needs to be focused on avoiding introducing seed-borne infection to the crop, and also using cultural control measures at sowing time that minimise aphid landings at the vulnerable early crop growth stage. Insecticide application as seed dressings and as foliar sprays is pointless and can even make things worse by agitating aphids making them move around more so infecting more plants. Also, varietal resistance to aphids does not help. This applies to all non-persistently aphid transmitted viruses like CMV, but not persistently transmitted ones.

A healthy seed source is a community effort

Management of CMV is a community effort. Buying in healthy seed stocks or maintaining a local healthy seed source isn’t just important on an individual farm basis as CMV is easily transmitted by aphid vectors across the fence. Like managing rusts, what your neighbour does matters too. Even though you may have dumped virus infected seed, transmission can still occur from other pulse or weed hosts on neighbouring properties.

When harvesting pulse crops infected with CMV hard grading of the seed to remove some of the infected seed (infected seed may be smaller in size) can help to reduce infection levels. Testing a representative sample of your seed stock is considered best management however if you are unable to get a representative seed sample tested prior to sowing, it is still important to know what virus infection level is present in your stored seed so you can make a decision about what to do with it. Stored seed can be tested in season so you know what you’re dealing with for subsequent sowings and you can begin to plan ahead to dump seed with high infection levels in favour of a healthy seed source. The only effective way to ensure a virus reservoir doesn’t build up is by “dumping” seed infected above infection thresholds (Table 1).

Table 1. Infection levels – rules of thumb

Recommendations for sowing

  • Sow seed with no or minimal infection levels.
  • Remove all weeds in the paddock and around the edges that may harbour aphids, with a two week window before planting the crop.
  • Sow later to minimise exposure to aphid flights (aphids tend to stop spreading geographically with the colder temperatures).
  • Sow at a higher seeding density and close row spacing to create a denser canopy that is less attractive to aphids.
  • Sow into stubble groundcover to reduce aphid landings.
  • Lupin growers can sow lupin varieties with low seed transmission ratings (Table 2)

Table 2: Lupin varietal seed transmission ratings.

Seed infection ratings: I=Immune, R = Resistant RMR = Resistant to moderately resistant MR = Moderately resistant MRMS = Moderately resistant to moderately susceptible MS = Moderately susceptible MSS = Moderately susceptible to susceptible S = Susceptible SVS = Susceptible to very susceptible VS = Very susceptible

Virus Testing Service

Seed testing services across Australia are listed below.

Further information

Acknowledgements

Piotr Tribecki, Agriculture Victoria
Jenny Davidson, SARDI

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