Sustainable disease control using weeds as indicators: Capsella bursa-pastoris and Tobacco Rattle Virus

Arable weeds are believed to sustain disease outbreaks of the potato crop pathogen Tobacco Rattle Virus (TRV), which is particularly well-known for the costly damage it may cause to potato tubers. We describe a TRV-specific TaqMan™ based molecular-diagnostic quantitative RT-PCR method which showed that ecotypes of the widespread and common weed Capsella bursa-pastoris (shepherd’s purse) are highly susceptible to TRV infection and may be suitable as indicator species of TRV presence in situ. Soils from two sites (S1 and S2), previously diagnosed as harbouring high levels of TRV, were the subjects of infection tests using C. bursa-pastoris and the susceptible model bait species Petunia x hybrida. TRV infection was only detected in all S1-soil, but in none of the plants grown in S2-soil. S1 soil had been treated annually with nematicide and herbicide, whilst continuing to cultivate TRV susceptible crops. S2 soil had been farmed for 5 years without the application of synthetic pesticides according to organic standards and had been sown with non-TRV susceptible crops in three out of the 5 years of the rotation. Our observations led us to question the current recommendations that: ‘Weed control is important. Organic practices and set-aside may facilitate the re-introduction of TRV and/or the increase the distribution of the virus within a field ’. We suggest that more effective and less environmentally damaging crop protection can be achieved using rotations that employ non-susceptible crops, in concert with management strategies that encourage crop-weed co-existence.