Cross-protection against pepino mosaic virus, more than a decade of efficient disease control

Cross-protection was first described a century ago. In practical terms, cross-protection consists of the pre-inoculation of plants with mild or attenuated viral isolates that protect the plants from subsequent infections with aggressive isolates. Experimentally, cross-protection has been shown to apply to numerous virus and viroid species infecting numerous plant hosts, but the number of cases in which it has been successfully used for the control of the damage caused by viruses in crops is rather limited. Significant examples include citrus tristeza virus and zucchini yellow mosaic virus, and more recently, pepino mosaic virus (PepMV). PepMV is a pandemic virus affecting tomato crops worldwide. During the first decade of the current century, a number of mild PepMV isolates useful in cross-protection were identified and characterized. Soon after, cross-protection programs against PepMV were implemented in the Netherlands, Belgium, Morocco, and Spain. These programs were easily adopted by farmers, as no other efficient control methods were available and the losses caused by PepMV were unbearable for tomato producers. PepMV cross-protection has been working exceedingly well during this time, although the potential interaction of the PepMV mild isolates with the recently emerging tomato brown rugose fruit virus (ToBRFV) may be threatening its viability. In this review, we describe our experience in deploying cross-protection against PepMV in Spain. In our view, this is an underexploited technology that holds promise, but when it is used and how it is used requires careful thinking and preparation, as not all situations are as favourable for the use of cross-protection as the PepMV pandemic has been.