Canker Development and Biocontrol Potential of CHV-1 Infected English Isolates of Cryphonectria parasitica Is Dependent on the Virus Concentration and the Compatibility of the Fungal Inoculums

Abstract

Biological control of Cryphonectria parasitica fungus, causal agent of chestnut blight, by virus infection (hypovirulence) has been shown to be an effective control strategy against chestnut blight in Europe and some parts of North America. The most studied mycovirus is the Cryphonectria hypovirus 1 (CHV-1) type species of the Hypoviridae family. To efficiently provide biocontrol, the virus must be able to induce hypovirulence in its fungal host in chestnut trees. Here, two different CHV-1 subtype I virus strains (E-5 and L-18), gained by transmissions, were tested for their hypovirulence induction, biocontrol potential, and transmission between vegetatively compatible (VCG) and incompatible fungal isolate groups in sweet chestnut seedlings and branches. Both strains of CHV-1 showed great biocontrol potential and could protect trees by efficiently transmitting CHV-1 by hyphal anastomosis between fungal isolates of the same VCG and converting virulent to hypovirulent cankers. The hypovirulent effect was positively correlated with the virus concentration, tested by four different reverse-transcription PCRs, two end-point and two real-time methods, one of which represents a newly developed real-time PCR for the detection and quantification of CHV-1.

Molecular genetic variability of Cryphonectria hypovirus 1 associated with Cryphonectria parasitica in South Tyrol (northern Italy)

“…Therefore, a high degree of genetic diversity of CHV-1 appears to be a natural progression in established European populations. In contrast, a low diversity was observed in newly established populations, such as in Great Britain (Romon-Ochoa et al, 2022 ).…”

Temperature Effects on the Cryphonectria hypovirus 1 Accumulation and Recovery within Its Fungal Host, the Chestnut Blight Pathogen Cryphonectria parasitica

“…The low prevalence of CHV1 (17 out of 350 isolates) and the naturally low virus concentration in these isolates (1.9 to 48.1 ng/µL of viral amplicon after RT-PCR, [ 15 , 24 ]) in England is most likely related to a mean monthly temperature below 7.7 °C between November and April and a mean of 15.2 °C the rest of the months, over the last 8 years [ 33 ]). In contrast, the higher viral loads in the warmer temperate areas of continental Europe [ 34 ], leads us to hypothesize that the concentration and hypovirulence of this mycovirus is proportionally related to higher mean temperatures around 25 °C [ 22 ], therefore the prevailing climate is likely to have a significant influence on its success as a biocontrol agent.…”

“…Fluorescent detection occurred at the end of each 53 °C step. The cycle threshold (Ct) value was calculated automatically using the LightCycler software with absolute quantification using second derivative maximum setting with 465–510 and 533–580 nm channel filters respectively for analysing the specific amplicons and the actin RNA quality control [ 22 ].…”

“…Recently highly CHV1-infected isolates of VCG EU-10 (dominant in London and ranging between 234.4 and 407.1 ng/µL of viral amplicon after RT-PCR) and VCG EU-9 (dominant in Devon and ranging between 343.0 and 612.8 ng/µL of viral amplicon after RT-PCR) were achieved by co-culture transmissions with CHV1-infected cultures from Switzerland [ 22 ]. The aims of the present study were: (i) to analyse the influence of six temperatures (5–30 °C, in 5 °C steps) on the mycovirus accumulation levels in those highly infected British isolates of C. parasitica ; (ii) to assess the relationship between temperature, mycovirus accumulation, and C. parasitica growth; and (iii) to determine the capacity of the virus to recover after each temperature treatment.…”

Forecasting of Airborne Conidia Quantities and Potential Insect Associations of Cryphonectria parasitica, the Causal Agent of Chestnut Blight, in England

“…Research is now focused on possible biocontrol using Cryphonectria hypovirus 1 (CHV1) on those sites where one dominant Vegetative Compatibility Group (VCG) [ 12 ] is present, and CHV1 has been detected. CHV1 from highly infected fungal strains from Europe has been successfully transmitted to uninfected English C. parasitica isolates of the same VCGs (mainly EU10 and EU9) and further experiments about the in planta and thermal behaviour, under controlled conditions, of those virus-infected isolates have been completed successfully [ 13 , 14 ].…”