Fundamentals of infectious disease in deciduous fruit trees. By Anna Mouton.
This has been the year of the amateur epidemiologist. Words like asymptomatic and co-morbidities have become part of everyday language. As they should be — if you are a deciduous fruit farmer. Read on to learn how lessons learnt from COVID-19 apply to your orchards.
Know your pathogens
Pathogen literally means disease-causing. The name of the pathogen is often different to that of the disease it causes. The coronavirus that causes the disease COVID-19 is called SARS-CoV-2. The trichovirus that causes apple chlorotic leaf spot, and plum pseudo-pox, and other diseases, is called apple chlorotic leaf spot virus.
So what is a virus anyway? A basic virus particle consists of a protein coat enclosing a set of genes. These genes are instructions for making the virus. Viruses hijack the cellular machinery of their hosts and turns this machinery to producing more viruses. Sometimes the host cell is destroyed in the process and disease symptoms develop.
A viroid is similar but different. A viroid is just a short loop of genetic material that is copied inside the host cell. Viroids are much simpler than viruses. They do not contain any genes and have no protein coats.
How about other pathogens? Bacterial cells are roughly ten to a hundred times bigger than viral particles and fungi are even larger. Bacteria and fungi are also far more complex than viruses.
Symptoms and superspreaders
Why do some people infected with SARS-CoV-2 become sick or die, while others are asymptomatic? One contributing factor is pre-existing conditions, and this factor also applies to trees.
The outcome of most infections depends as much on the host as on the pathogen. Hosts may be less affected due to their genetic makeup or their overall health. Trees that enjoy ideal conditions are less likely to show symptoms of infection than trees that are under stress. This may be why diseases like plum marbling are more noticeable in some years than others — it isn’t because the trees lose the infection.
We all know that people infected with SARS-CoV-2 can look perfectly healthy while spreading the virus. The same applies to trees. They can look perfectly healthy while infected with all manner of pathogens. And the problem with trees is that they don’t wear masks and they don’t socially distance.
The worst case is when an asymptomatic tree is a source of propagation material. Some pathogens, including most viruses and viroids, spread through the entire tree after infection. If a tree in a foundation unit is infected, it could lead to a mother block being infected, which could in turn lead to thousands of infected nursery trees — think superspreader.
A quick glance at the routes of transmission on the facing page and you can see even more potential for infected trees acting as superspreaders. There are no vaccines for trees, so how can we keep orchards safe?
The most important step is to establish orchards with nursery trees that have been certified disease-free. This isn’t perfect, because sometimes new diseases emerge, and infected trees are propagated before testing is available. But planting certified trees sets the orchard up for success by preventing the introduction of several serious pathogens.
Emerging diseases are a fact of life for people and for plants. Understanding how pathogens spread is key to winning the war — whether on plum marbling or on COVID-19.