Most horticultural crops will face some sort of plant health issue at some point in their lifespan. Annuals are basically in a race to the finish, in a strange “be faster or nimbler than the guy behind you (because there is a bear chasing you)” sort of effort. Perennials will face challenges as they establish, and then again and again as they mature and age.
Other than catastrophic physical damage that can come suddenly (e.g. with severe storms), most issues in a field or orchard do not “just appear”. Problems develop at different rates. While it is possible for plant health to decline rapidly for some issues, with plants going from healthy to dead in a matter of days, this would be the exception, not the rule. It is rarely quite that quick, and there will be earlier indicators that all is not perfect in paradise. As plants grow, signs that plants are not operating at 100 percent may start to appear. Many issues will stew away gradually within the canopy or the field, until suddenly there is no ignoring the issue. Most plants will put up a range of warning flags, which are useful, provided you are watching for them.
Establishing a Baseline
The first element of this proactive process is establishing a baseline. Much of the art of diagnosing issues in a yard or garden is about recognizing change. And you cannot see a difference if you have never established a baseline to compare things to from the start. You need to understand what “normal” looks like in the different growth stages of your plant.
For a perennial plant, what does normal look like in a dormant plant? What about in full flower? What is a normal or expected level of production or yield? For annuals, what does the plant look like as it emerges, expands, and matures? For all plants, what are normal colours, shapes and sizes?
At first, you will need to create a virtual or theoretical baseline in your mind’s eye, with an experienced baseline coming with time. As a growing season progresses, you will continue to reestablish that baseline as the season progresses. In short, you must watch your plants.
Warning Signs (a.k.a. signs and symptoms)
Missing Plants
In my experience, once most plants are seeded, assuming that conditions are averagely warm and moist, you should start to see plants emerge within less than two weeks. If you are watching where you planted things and nothing is coming up, or you see big gaps once things do emerge, that might be an indicator that something is wrong with the germinating and emerging seedlings. There is always variability in seed health and vigor, but human error can lead to differences in depth, spacing and overall accuracy, so give it a few days before raising the alarm. You can dig around gently and see if you pick up any clues.
Disappearing / “Shrinking” Plants
Once plants are up (which you noted diligently), watch for what happens to them. Watch for whether they stay there and continue to grow. If you suddenly notice that plants are missing from one day to another, or plants have collapsed or start to waste away, this might be an indication that something is either preying on them (e.g. cutworms) or something is attacking the seedlings (e.g. soilborne fungal pathogens).
Size differences between adjacent plants can be a good clue that something is amiss. We can use my small-scale Brussels sprouts patch for an example. The transplants that I used came in the same container, which I split and planted side by side in the same bed. They both get water when I water. They appeared equal in almost every way, until I decided to fertilize one day. I applied a similar amount of low analysis fertilizer, followed by some water for both. However, within a few days, there was a significant change in the plants. One set practically leapt out of the ground, growing quickly and happily. The other set appeared to droop, and growth has stalled, for all intents and purposes. The plants have poor colour and look like a wet dog, for lack of a better descriptor. Further investigation will be required to sort out the cause, provided they now survive the recent hailstorm (ouch). But take the desired lesson from this. Watch for size changes or sudden deviations from normal, and you might catch an issue early.
Size differences between Brussels sprouts
Photo by Robert Spencer
Colour Changes
Plants have limited ways of communicating their issues to us. But one of the ways that they “speak” to us is through colour change, specifically changes that occur out of season. If you see a gradual fading or weakening of colour, gradually turning more and more yellow, this can be an indication that the plant isn’t getting the nutrition it is needing, either due to damage below the soil surface, or due to deficiencies in the soil. If you see a sudden colour change, such as a shift from green to purple or red, this is another signal that things are not functioning properly. Investigate the roots and determine their health. A plant could be supplied with everything it needs, but without viable roots, it will not matter at all.
Where the colour change occurs is also important. Are the older, lower leaves turning colour, or is it the young, new leaves? Is everything changing colour, or just specific parts of the leaves. For example, are the veins staying green, while the rest of the leaf turns yellow?
Are there any other abnormalities showing up that are associated with the colour change? For example, are leaves turning a bright red in certain spots, and those same spots are crinkling, bubbling, or twisting? Are the colour changes associated with spots, holes, lesions, or other points of injury?
Bite marks and holes
If you start to see little holes or missing edges from your leaves, that is likely an indicator that something is chewing on the plant. Look for the culprit under the leaves, in the top layer of the soil, or right on the leaves. The nature of the damage, as well as the pest life stages that you might find, will give you the clues to identifying and dealing with the culprit. The timing of when the damage appears can also offer valuable clues.
Crucifer flea beetle damage (shotholes) on Cole crops
Photo by Robert Spencer
Oozes, swelling, sticky and fuzzy stuff
If there is an accumulation of sticky fluid on some part of the plant, either associated with a wound or a hole, you might find something infecting or infecting that location. You might find a colony of aphids (guarded by ants) pumping out sweet “honeydew” or plant juices during their feeding, or you might find some light webbing on the undersides of leaves, along with sand that moves (mites).
Sometimes, you will find a point where the leaves, stem, or some other plant part have a strange growth or swelling on them. It might be tiny or could be quite sizeable. If it is not really obstructing growth, you can probably ignore it. Or, if you want to unveil the culprit, you could cut it open and see what it inside. In many cases, you will not find anything, simply because the swelling is a response by the plant, stimulating tissues to divide rapidly, essentially cordoning off the point of infection or attack.
Fungal pathogens typically have some sort of visible stage where they are growing on the surface of the plant, whether it be spores or other structures. It might have a different colour and may or may not be associated with rotted tissues. If you see fuzz or growths, look closer for the possible cause.
Saskatoon berry / Juniper Rust on Saskatoon berry leaf
Photo by Robert Spencer
Twists, Curls and Deformities
You can sometimes see that a plant is being affected by something when the tissues start to act strangely. You might see them twist and curl or swell. If the plant suddenly starts growing oddly, look closely for what might be going on.
Consider what might have happened in the vicinity of the plant (or plants, if more than one is affected). Was an herbicide applied somewhere nearby? What is the nature and mode of action of that product? If you are using a piece of ground for the first time, consider what might have been applied there in the previous 2-3 years.
Herbicide injury in potatoes
Photo by Robert Spencer
Conclusions
While not all problems can be solved when you find them, if you notice them early, you have a great chance of choosing a course of action or treatment that might minimize the impact (short and long term) to your crops and plants. So, establish a baseline or “normal” and then keep a watchful eye out for potential or expected pests and problems. Watch for the signs that plants put out.