Just like we get colds and flus and stomach bugs, plants can suffer from disease as well. Not the same diseases that we get obviously, they have their own long list of infections to deal with. Don't worry, plants generally don't pass their diseases onto us, but plant diseases can certainly spoil your fruit and veggie crops, and at worst, they have the potential to completely wipe out entire species or cultivars of plants if they get out of control. Look up Gros Michel banana and Panama Disease to learn about one instance in the not too distant past of a plant disease almost completely wiping out a cultivar of banana from the face of the planet. This is why biosecurity in Australia is so important. Some of our native species in particular, having been isolated from the rest of the planet for long, have no immunity at all to some of the plant diseases that exist in other countries.

Yes, immunity is a thing for plants as well. We survive the common cold because we have an immune system that can cope with it, but that hasn't always been the case. The common cold has killed off many people of isolated nations in the past when they were first exposed to it by sea faring explorers that carried new diseases to other lands. Those that survived passed their resistance to those diseases onto their children and thus the vast majority of humanity has evolved to tolerate many diseases as minor inconveniences. That being said, someone with a weakened immune system can still be killed by a common cold, and of course there are still some dangerous diseases to be wary of, but overall, we've got it pretty good compared to people that lived a few hundred years ago when the idea of microorganisms wasn't even known about and washing hands wasn't seen as something important to do.

Just as we can have a weakened immune system, so can a plant. Like us, a healthy plant is much more capable of fending off disease than a plant that is weak. Too much or not enough water, insufficient or too many of the wrong kind of nutrients, the wrong kind of soil or or soil pH level, and too much or not enough sunlight, are the most common causes of plants being weakened and falling victim to disease. That being said, a perfectly healthy plant can also be overrun by disease if exposed to enough of it.

What do I mean by "exposed to enough of it"? Well, you may have heard with all the covid-19 information going around something about a thing called viral load or viral shedding. If we're exposed to one or two little particles of a disease microorganism (virus, bacteria, or fungi/yeast), our immune system will attack it and win, and we'll never notice. However, if we're exposed to too large an amount of it, it can be too much for our immune system to cope with and we get sick. How this happens is all about numbers and exponential growth. Imagine our body is our house, and we, the human residents, are it's immune system. A cockroach gets into your house. You find and kill it before it lays an egg, cockroach problem solved. Now imagine if a thousand cockroaches get into your house all at once. You can't find them and kill them all before some of them start laying eggs. Each egg hatches and creates a thousand more cockroaches. It wouldn't take long for your house to become infested with cockroaches and leave you with no choice but to call pest control for help. That's pretty much what happens with disease in our bodies verses our immune system. We can breathe in or swallow one or two little disease particles from the air and we'll probably be just fine, but if someone with a high viral load disease sneezes or coughs in our face, we're going to be bombarded by millions of those little disease particles. Bacteria and viruses multiply quickly and if there's too much of them to start with, they can quickly overrun our immune system, and if our immune system has never seen a particular microorganism before, it can take a while for it to figure out how to kill it, and by the time that happens it can be too late for us.

Fortunately for us humans, we have vaccinations and antibiotics to deal with most diseases. Vaccines teach our immune system what different viral diseases look like so it knows how to attack them efficiently before the virus kills us, and antibiotics attack bacterial diseases for us like a little support army for our immune system. Plants don't have it quite so lucky as us, but fortunately for them, they have us to look after them, which gives them a much better fighting chance.

We do have commercial sprays and treatments that will kill off plant diseases, and that's great, but prevention is better than cure for plants as well. Just like we have the ability to avoid a lot of disease by taking simple precautions such as washing our hands, we can also give our most vulnerable plants protection against diseases as well. For a start, we can pay attention to and obey biosecurity laws. We can also lower their exposure to diseases that already exist in our area which they're prone to, so their immune systems can fight the diseases off. The best way to do this is by crop rotation.

Now crop rotation isn't going to work for a fruit tree or any other perennial plant. Long living plants are more likely to be weakened by being dug up and moved around, so we don't want to do that. They aren't prone to the same diseases that are inclined to wipe out annual crops. There are ways to help them avoid certain diseases, such as making sure they have plenty of fresh air moving through their branches by pruning as one example, and keeping them healthy in general. But for the purpose of this article, we'll be talking about annual plants that are grown, picked or go to seed, die back then are regrown the following year. Such as potatoes.

Potatoes are the world's most infamous example of plant disease. In 1845 the Irish Potato Blight started and it officially ended in 1851. This was caused as much by poverty and political repression as it was the potato blight disease itself. Originally farmers would grow potatoes annually as well as other crops, and would usually rotate the crops even if just for the sake of having a variety of food available. However, poverty and desperation for more quickly grown food forced farmers to replant potatoes immediately in the same plots repeatedly year after year, and then disease set in and destroyed these so heavily relied upon crops resulting in them having almost no food at all. It was a genuine disaster that could have been completely avoided with the right knowledge and planning.

So how did the disease come to infect the potatoes? It goes back to numbers and exponential growth. One crop of potatoes planted in a plot of fresh clean soil won't be exposed to very much if any of the microorganisms that cause disease. The disease is there, just in such small numbers that the potato plants can resist them. If potatoes are again planted in the same patch of soil the year after, they're going to be exposed to a little more of that disease, because the disease has been growing as well. This second crop will probably survive without any problems also. The third crop might not fare quite so well. They'll probably survive, but may look a little sickly because now the disease causing microorganisms have increased in number to the point where the plants are struggling to defeat it so easily. As each year goes on, with more potatoes being planted in the same soil, the more disease is present in the soil, and the harder it will be for the potatoes to beat it, and eventually an entire crop is going to fail and die. Crop rotation can allow that situation to be completely avoided.

So how does crop rotation work? Some articles and information out there make it seem really complicated, but it's pretty simple really. If after a crop of potatoes is done for the year, something completely different from a different plant family is planted in the soil instead, or if the soil is allowed to rest and nothing is planted for a while, the disease will have no potatoes to attack and feed off and the microorganisms that cause the diseases will die back. Then potatoes can be safely planted in that soil once again. This is how and why crop rotation works. We plant potatoes, once the crop is done, we might plant carrots, once that crop is done we might plant broccoli, then beans, and after all that, we can go back to potatoes again. This not only gives us a variety of things to eat and allows us to always have something growing according to the season, it also helps keep the soil and the plants that we grow healthy and free from diseases.

There's a lot of debate about how to best do crop rotation and how long each rotation should last, what order different plants should be planted in, and it can all sound too complex to even attempt for many people, but I don't think there's one rule to suit them all. With so much variance in what plants are available in different places, the different seasons and climates, whether or not we fertilize and refresh the soil with manure and compost, and even what foods we like to eat, all play a part. Some people prefer to have their crops on a 2 year rotation, others want to play it very safe and go for up to 6 years, but a lot of it will come down to one simple question: how many garden beds do you have? If you have 3 garden beds, then you could grow potatoes in one bed one year, and in another other bed the next year, and the 3rd bed the year after, which would mean with a 3 year rotation - which is plenty of time for disease avoidance in most cases - you could still grow a crop of potatoes every year. Consider this if you're about to renovate your garden, because depending on what you want to grow and how often you want to grow it, you might be better off with several smaller veggie gardens rather than one large one.

Another way to help minimise disease with annual crops in some cases is with companion planting. Beans and peas will add nitrogen to the soil. Marigolds will keep away root knot nematodes. Some plants will attract beneficial insects, whilst other plants may work to attract harmful insects to themselves and thus keep them away from a precious crop. All these types of things can strengthen plants and increase their ability to resist disease. Companion planting can be a little bit hit and miss and can seem complex, but it's simply about creating a beneficially healthy mini-ecosystem where all the plants can thrive together with the insects and such. Creating a healthy biodiverse micro-environment that strengthens the plants growing within it is going to go a long way towards protecting them from disease. On that note however, companion planting with the wrong combination of plants can result in the exact opposite effect. You'd need to get some solid information about exactly what will go well with what you want to plant for your climate and conditions given what you have available in your area, and that is too long a topic to cover for everyone here. Maybe other people can write a series of articles about companion plantings that work in their area with the plants that they're growing?

When planting out young seedlings, or if direct sown a bit after your seedlings have emerged and are getting settled, there are some things you can do with some plants to help give their immune systems a little extra kick. I understand that there's a thing that's been proven to work with strengthening the immune system of tomatoes; feeding into their soil, of all strange things, a little bit of dissolved aspirin. Which begs the question, how well would it work to mulch their soil with the bark from a willow tree, seeing as that's where aspirin comes from? Using some commercial products such as a seaweed based fertilizer is reputed to be like giving some plants a dose of vitamins, or a vaccination injection. How well it works, well, I suspect a lot of it is hearsay, but a lot of gardening tips are just that. If it's been working for so many people for so long, there may well be something to it.

Some people will say to never put a diseased plant into compost. That's true, if you're wanting to get your compost into the garden quickly, which let's face it, most of us do. Diseases are living things, and just like disease living in soil, if you leave your compost for long enough, the diseases will eventually die off and then your compost will be safe to use. Be aware however that with some diseases that can take up to a couple of years to happen, so it may be best to heed that advice about not using diseased plant matter in your compost unless you know exactly what diseases you're dealing with and how long it'll take for them to cark it in the compost pile.

Lastly, well, last that I can think of at present anyway, is keeping your gardening equipment clean. Putting a plant into a second-hand flower pot that is covered in dirt and grime from someone else's garden could bring disease into your yard, so clean it first. You wouldn't want a surgeon cutting you with a dirty scalpel, so don't cut or prune your plants with dirty secateurs. Plants are living things. They might not look like us, they might not breathe, eat, drink, grow, and reproduce the same way we do, but they still do all those same things in their own way. They can get sick, just like we can, and they can be protected from diseases with some simple good hygiene practises, just like we can be. Rotate your crops, wash your hands, get out there and, as Mark says, "get into it" and keep yourself and your garden healthy.

[Photo credit: https://es.wikipedia.org/wiki/Archivo:Aceraceae-Plant-diseases.JPG]