Virus diseases in Carrots: The UK experience Adrian Fox Fera Science Ltd
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Carrot virus research in Europe (up to 2012) Scotland: Carrot Motley Dwarf complex Hogweed virus 6 (and others ) Netherlands: Carrot Motley Dwarf complex HV6 (Carrot yellow leaf virus) Parsnip yellow fleck Germany: Carrot Yellow leaf virus Carrot thin leaf virus England: Parsnip Yellow fleck virus Carrot Red leaf virus Carrot red leaf associated viral RNA
Current state of Knowledge Carrot necrotic dieback virus (Formerly Parsnip yellow fleck virus Anthriscus strain) Important early season disease Sporadic outbreaks Associated with seedling death Stunted, misshapen roots in mature plant infections Needs Anthriscus yellows virus (AYV) for transmission Carrot is not a host AYV therefore NO onward transmission in carrot crops Source: AAB DPV394
Current state of Knowledge Carrot Motley Dwarf disease Three pathogen complex Carrot red leaf virus (CtRLV), Carrot mottle virus, Carrot red leaf associated viral RNA Needs CtRLV for aphid transmission CAN be spread from carrot to carrot Causes reddening of leaves and mottle Thought to cause splits in roots called kippering
Rooting out the cause of internal browning
Viral browning of carrots (HDC FV382a) Conventional PCR of affected & unaffected carrots showed no association with expected viruses Stocks of carrots rejected due to the presence of internal necrosis linked to viruses. Real-time PCR demonstrated a strong association between CYLV and necrosis, suggesting that removal of this virus would reduce necrosis by 96% samples positive 100 90 80 70 60 50 40 30 20 10 0 affected unaffected PYFV CRLV CRLaV CMoV CYLV Negative Virus MiSeq sequencing identified Carrot yellow leaf virus in necrotic samples. Sequencing also detected a Closterovirus (CtCV-1) and a Torradovirus (CaTV-1) plus several other viruses which were new to science. Adams et al. (2014) Carrot yellow leaf virus Is Associated with Carrot Internal Necrosis. PLoS ONE 9(11): e109125. doi:10.1371/journal.pone.0109125
AHDB FV382b and Defra projects : understanding the novel viruses
Field sampling 2014 Field Margin and Road N 197 208 219 230 241 252 263 274 285 297 308 198 209 220 231 242 253 264 275 286 298 309 199 210 221 232 243 254 265 276 287 299 310 200 211 222 233 244 255 266 277 288 300 311 201 212 223 234 245 256 267 278 289 301 312 202 213 224 235 246 257 268 279 290 302 313 203 214 225 236 247 258 269 280 291 303 314 204 215 226 237 248 259 270 281 292 304 315 Hedgerow 205 216 227 238 249 260 271 282 293 305 316 206 217 228 239 250 261 272 283 294 306 317 207 218 229 240 251 262 273 284 295 307 318 CYLV 23% CTV 25% DUAL 9% Total Virus 39%
Relative virus incidence Carrots 2015 (Two regions) 250 CMD CYLV CaTV CtCV1 200 150 Virus % 100 50 0 Stamford Bridge Stamford Bridge Late Sutton of the Forest Elvedon Estate Highman 'Healthy' Highman 'Infected' Yorkshire (North) Norfolk (East)
Sources of virus? Cow Parsley (Anthriscus sylvestris) Hogweed (Heracleum sphondylium) Ground Elder (Aegopodium podagaria) Hemlock (Conium maculatum) Rough Chervil (Chaerophyllum temulum) Alexanders (Smyrnium olusatrum)
Warwick Crop Centre weed samples 1555 samples UK Wide sampling Collected over 3 years (2010-2012) 10 species 20-30 sample locations per year Previously tested for CtRLV, PYFV, AYV RNA extracts retained at Warwick (-80 o C) PYFV and CtRLV findings sequenced to look at population genetics Many wild hosts, few carrot Cow Parsley only Carrot/Wild Carrot Carrot red leaf virus from Defra IF0188
% Proportion of positive virus cases Relative proportion of viruses in weeds 70 60 50 40 30 20 10 0 2011 = 472 virus cases 2012 = 174 virus cases CtRLV PYFV AYV CYLV CtCV-1 CaTV 2011 2012
Viruses detected by host species 60 Percentage infection (%) 50 40 30 20 10 0 Alexanders Cow Parsley Ground Elder Hemlock Hogweed Rough Chervil CRLV PYFV AYV CYLV CtCV-1 CaTV
Aphid Transmision of CaTV-1 and CtRLV
Aphid vectors of carrot viruses Virus Vectors? Transmission mode? CMD: Carrot red leaf virus (CtRLV), Carrot mottle virus (CMoV) and carrot red leaf associated viral RNA (CtRLVaRNA) Parsnip yellow fleck virus (PYFV) requires helper -Anthriscus yellows virus (AYV) Carrot yellow leaf virus (CYLV) Carrot torrado virus (CaTV) C. Aegopodii M. persicae C. aegopodii C. pastinacae C. aegopodii C. pastinacae C. theobaldi M. persicae M. persicae C. aegopodii Circulative, persistent non-circulative, semi-persistent non-circulative, semi-persistent Carrot closterovirus-1 (CtCV-1)???
Sources of carrot viruses. From Cow parsley AYV PYFV Aphids/Eggs on winter host Aphids Migrate Pick up Virus Transmit to Carrot From Carrots CtRLV (CMoV) (CtRLVaRNA) CaTV-1?
Knowledge gaps Relative influence of peach-potato aphid in carrot crops Aphicide resistance status for willow-carrot aphid How much does this species contribute in the field in the UK? should we worry about this.? Can we spray the right chemical at the right time? Epidemiology of CYLV and CtCV-1 Is crop to crop transmission the major concern (e.g. CMD)? Is weed to crop transmission the major concern (e.g. PYFV)? When is the virus coming in? How long is it being spread within crop throughout the season? Can we tailor crop management practices to minimise virus risk?
So what does this all tell us? 250 CMD CYLV CaTV CtCV1 200 PYFV possibly underreported Virus% 150 100 50 0 Stamford Bridge Stamford Bridge Late Sutton of the Forest? Elvedon Estate Highman 'Healthy' Highman 'Infected' due to sampling time Highest incidence viruses may come from carrot as a source Is the source overwintered carrots or volunteers? Virus control appears to be breaking down. Length of season? Vector resistance? Limited range of control measures?
The principles of plant virus management : Plant clean seed Grow in absence of vectors Grow in absence of virus reservoirs Isolate from similar crops Use resistant (or tolerant?) varieties
Acknowledgements Dr Derek (Dez) Barbara (1948-2012) Zurine Rozado s Carrot torrado virus PhD work conducted under UK Government-Defra Plant Health Capability funding Initial carrot necrosis work and Carrot yellow leaf virus work funded by AHDB-Horticulture (FV382a & b) Fera Staff in Virology, the MTU and Applied Entomology Staff of Warwick Crop Centre