By: Héctor García O., Co-Founder and General Manager of Diagnofruit Laboratories Ltda, SOCHIFIT and AMICH Member. [email protected]
Without a doubt, the 2020-21 season has been marked by viruses. SARS-CoV-2 has had us living a true nightmare from which we have not yet woken up. A FakeNews that appeared in China, which indicated a contamination of cherries with coronavirus particles, generated an extremely negative reaction on sales, several days without movement and not only of Chilean fruit, which could only be reversed after a public-private strategy. To date, we do not have clarity on the start of this false news, perhaps it could have been inspired by the surveillance work of the Chinese state on the presence of plant viruses or phytoviruses in our cherries, which caused confusion; for this reason we must pay maximum attention to these phytopathogens, since they are not only possible generators of direct losses in our orchards, but they can also be generators of tariff barriers in our destination markets.
What do we know about viruses in cherry trees in Chile?
Research on viruses in stone fruit plants in Chile began several decades ago and probably the detection of Plum pox virus (PPV) in 1992 has been the most relevant finding to date, since this is considered the virus with the greatest impact on Prunus production, in addition to being considered a quarantine pathogen in important markets, for example China; however, as we will detail later, this phytovirus has not been detected in cherry trees in our country.
With the exponential growth of cherry plantations in our country, research into this species and the generation of diverse projects have become relevant, for example, to obtain virus-free plants, which has allowed generating basic information on the phytosanitary status of the species.
In October 2011, after collecting symptomatic cherry leaves from the O'Higgins and Del Maule regions, it was possible to detect Cherry necrotic rusty mottle virus (CNRMV) for the first time in Chile in 6 of 21 samples, a report that was also the first for South America according to researchers from the Phytovirology Laboratory of the University of Chile.
The leaves showed brown angular necrotic spots, the centre of which often fell off, which can be confused with ammunition or Pseudomonas attacks. At the same time, analyses were carried out for another common cherry virus, Cherry green ring mottle virus (CGRMV), where 10 of the 21 samples were positive and in 5 of these the infection was mixed.
Years later, in the spring of 2016, a new survey was carried out on 19 plants, although on that occasion more in-depth genomic analyses were carried out, allowing an exploration of all the viral genetic material present and then comparing it with databases, allowing the identification of the widest range of phytoviruses present in the samples (HTS analysis).
In this way, 1 sample was confirmed positive for Plum bark necrosis stem pitting-associated virus (PBNSPaV) and 14 for Cherry virus A (CVA), showing a prevalence of 74% of this last pathogen. Both detections constitute the first report in Chile and South America of PBNSPaV and CVA in cherry trees.
Almost at the same time, in surveys carried out between 2015 and 2016, an important pathogen was detected in Chile, probably at the level of importance of PPV, corresponding to the virus that causes Little cherry disease (LChD). However, LChD has two variants described to date, Little cherry virus 1 (LChV-1) and Little cherry virus 2 (LChV-2), with variant 2 being the one related to loss of productivity and fruit quality; LChV-1 would only cause symptoms in some susceptible varieties and in others it would cause minor symptoms or simply no symptoms.
Fortunately for our industry, the detection in Chile was positive only for LChV-1, which obviously should not stop us from worrying, since its worldwide spread is due to contaminated plant material, considering that to date there is no known vector, which should put us on alert for new introductions.
Climate Change, Multi-infection and Productive Management
In many cases, even though there are viruses infecting plants, they do not generate identifiable symptoms that we can associate with their interaction with the plant. This may be due to both the viral species and the tolerant varieties or rootstocks, which can efficiently avoid the harmful effects of many of the viruses mentioned.
But the state of equilibrium described is increasingly broken, fundamentally, in our view, by three factors: Climate Change, infections of several viruses in the same plant and yields at their production ceiling.
In the last 3 summers, heat waves have multiplied exponentially, and we can even observe them in spring. The stress caused to the plants, although it is difficult to quantify from one season to another (the presence of double fruits can be good evidence), would cause greater susceptibility and the appearance of symptoms associated with viruses in the following spring; in orchard sectors where management is poor, we have observed symptoms such as straw-colored spots on fruits, associated with viruses such as Prunus necrotic ringspot virus (PNRSV) or the described CNRMV and CGRMV (Photo 1). The ease with which these symptoms appear is also associated with the entry into full production of our plants, yields that have also been high in recent years, a situation that has occurred more and more quickly and steadily in Chile, due to the high technical level achieved by our industry.
As already described, the presence of several viruses in the same plant is a fairly common situation and we have been able to demonstrate this in the project led by Diagnofruit on Cherry Plant Death, where we explored, with the collaboration of Dr. Alan Zamorano, plant pathologist from the University of Chile, various etiological agents that generate serious diseases for our plantations. The first results show that 30% of the plants analyzed have more than 1 virus (Graph 1). This fact could also be a precursor to more aggressive symptoms; a recent report in Japan describes the presence of at least 6 viruses in plants, among which LChV-1, LChV-2, CNRMV, CGRMV, CVA stand out, which clearly under a stress scenario becomes a time bomb.
Cherry trees and PPV, different races and a limited distribution
Plum pox virus is the causal agent of Sharka disease, the most serious virus affecting Prunus. It is spread in the field through aphids in a non-persistent manner, which means that with just one test bite on an infected plant, the aphid is able to transmit the virus to a second healthy plant.
Officially, there are at least 9 PPV variants, described as races (D, M, Rec, T, An, EA, W, C, and CR). Of the most common, Race M is the one that causes the most severe symptoms and Race D, the mildest forms of disease; fortunately for our industry, to date only Race D is present in Chile (disease under official control since 1994 according to Resolution 796 of the Agricultural and Livestock Service and all its modifications, Resolutions 534 and 4906) and fortunately for cherry producers, Race D would not affect this species.
Of the 9 PPV races, 3 are associated with disease in cherry trees, the so-called PPV-C (Cherry), PPV-CR (Cherry Russian) and PPV-CV (Cherry Volga); the last race was recently proposed, which would generate a total of 10 variants. The CR and CV races have only been detected in Russia. PPV-C was initially identified in countries of the former Soviet Union, such as Moldova, Belarus and Russia, and was later detected in Romania, Hungary, Croatia and Germany. Most of these detections were made in individual trees within orchards, therefore, the epidemiological significance of the virus in cherry trees is still unknown.
In conclusion, the fact that a FakeNews has such an impact on sales in a market and that these are stopped from one day to the next, should keep us alert about any other similar situations. Therefore, the surveillance of viruses in vegetables, which would be the beginning of the confusion in China, is vital to maintain a trade that allows the sustainability of our industry and to continue being an agro-food power; evidence of this is the absence of PPV and LChV-2 in our cherry trees. The SAG is the official surveillance entity in Chile, however, the private sector must do everything possible to maintain an ecosystem free of pests and diseases, following the established regulations, promoting new guidelines and supporting local research from basic to applied science.
