Resistance to Cypress Canker Disease in Italian cypress has desirable effects on disease epidemiology, but may fail against novel genotypes of the pathogen Seiridium cardinale

Prevention is hailed as the only successful and cost-effective approach to control the introduction of exotic forest pathogens, and several international policies are in place to avoid such introductions. However, once a pathogen has been introduced and is widely spread in a novel range, regulations are relaxed. Concerns have been voiced about the detrimental consequences of the introduction of multiple genotypes of a pathogen, but evidence about such consequences has rarely been documented. Cypress Canker Disease (CCD) is a pandemic lethal disease of plants in the family Cupressaceae caused by the fungal pathogen Seiridium cardinale (W.W. Wagener) B. Sutton & I.A.S. Gibson (Sc). Sc is native to California, where populations are genetically diversified, while widely spread CCD outbreaks in the Mediterranean basin have been caused by the introduction of a single genotype. Resistant plant clones have been selected in Italy based on long-term field tests using a representative Mediterranean Sc genotype. Here we use a 90-day greenhouse trial on 419 21-month old grafts to test the response of four putatively resistant and three putatively susceptible clones of Italian cypress (Cupressus sempervirens L.) against one Mediterranean and five Californian fungal genotypes. By measuring mortality, size of the lesion caused by the fungus on the stem, and fungal sporulation, we ask the following questions: (1) Does the fast greenhouse assay confirm results of the extended field trials; (2) Is resistance confirmed for all putatively resistant clones when using a broad representation of the pathogen; and, (3) Does resistance have the potential to reduce transmission rates of the pathogen, thus potentially slowing down the CCD epidemic? Results indicated that: (1) Lesion size, but not seedlings' survival, can be used as metric to measure resistance in small grafted ramets; (2) One of four putatively resistant plant clones was considered susceptible at the end of the trial; and, (3) Sporulation was significantly lower in resistant than in susceptible plant clones. Based on these results, we conclude that resistance may be a viable disease control approach in this pathosystem due to the effect it has on both plant survival and fungal sporulation. However, we also conclude that resistance can be considered robust and effective against the pathogen species in its entirety, only if tested using several fungal genotypes. This study is one of the first to show that multiple introductions of a pathogen could break down host resistance and have a measurable detrimental effect on native ecosystems, suggesting that international policies should be revised accordingly.