When I was a kid, I used to love “The Just So Stories” by Rudyard Kipling. It isn’t so much that the stories themselves make all that much sense (since they don’t, logically), but I did love the creative and interesting way that natural phenomena are described to explain how different animals look the way that they do.

I can still hear my grandfather (who had a distinctive, deep, scratchy voice) reading to me how the leopard got his spots, and the elephant got his trunk. Looking back at them now, in the context of my professional life, I can appreciate how important an accurate description is, since that affects our perception of something.

Descriptions are key to diagnosis

Over the years, I’ve had many calls that started off with the words, “I’ve got a blight on…”. Such a description often left us at a point where the lack of information meant that an answer was a long way off. To adapt the immortal words of Crocodile Dundee (played by Paul Hogan), “That’s not a blight … this is a blight!”

When it comes to deciphering the mysteries of plant health and diseases, providing an accurate description of the symptoms that are showing up will go a long way to understanding what is causing the plant health problems.

Similar to insect pests, it is important to understand “what is what”, and in some cases, “why is why”. It is important to know the different terminology of diseases and understand the relationships between the various causal agents and their activities.

The various symptoms and signs that are observed are indicators or clues as to the cause of the problem. There is a host of descriptive terms associated with various causal organisms, which, when used correctly, can help to narrow down the list and focus in on of culprit(s) during the diagnostic process.

The (Basic) Anatomy of Pathogens

You could make an entire lifetime or two of studying what plant pathogens are made up of and look like. But, for the purposes of this article, here are some of the basics.

Fungi typically have two different types of growth. They can have their spore-producing parts (which can be quite variable) and then there is their vegetative type of growth. When it comes to spores, these can be sexual or asexual (conidia), but what you need to understand is that this how fungi move around from place to place, other than when they are physically carried on a host. You can identify fungi by how their spores look, but typically, you aren’t going to have the tools to do that outside of a lab. However, you can learn about the fungus by what it’s spore-producing/bearing structures look like, and maybe a bit about it by the colours of the masses of spores.

Fungal vegetative growth is called mycelium, made up of strands or strings of cells. I either think of them as looking a bit like roots or vines, but that isn’t entirely accurate. Fungi can spread out their mycelial growth to form mats, compress them together to form sclerotia, or they can use them to colonize tissues. Some fungi only produce mycelium, whereas for others, it is a base level of growth before the spores start flying.

I have definitely oversimplified fungi, but there are so many different types out there.

Bacteria are made up of many, many cells. They don’t move on their own but are carried in some way to their hosts. They typically need a hole or a wound to get in, but once there, they settle in and colonize the tissues, spreading outward from there. They can be noticeable by the symptoms that they cause, as well as by their oozes. Bacteria also stink, as a function of the secondary metabolites that they produce. Bacteria can be standalone invaders, or opportunistic pathogens, arriving once the party has started, but doing plenty of damage once they are there.

Symptoms

Symptoms are the terms that describe a plant’s response to attack by an organism, stress, or other factor. In some cases, they are essentially the flag that the plant sends up to signal that something is wrong. As things develop, they typically relate to how an attacker is penetrating the plant’s defenses and actively colonizing the plant tissues, and/or how the plant is adjusting to combat the invader. Once you see the flag, you can look more closely to see if you can learn anything more about the pathogen (or whatever else might be causing the issue).

When it comes to the terminology involved, the description is important because it gives you clues as to the rate of attack, the amount of plant tissues being attacked, and might give you clues as to the identity of the culprit. Think of symptoms as how an eyewitness to a crime might describe the perpetrator. “They had brown hair, a birthmark shaped like a banana on their right cheek, and glasses…”. And, as you use different terms, you can layer on more description and detail as you go. “It was short brown hair, with designs carved into the sides…”

NECROSIS is a term used to describe the death of groups or patches of cells or tissues, typically used to describe a situation with black/brown dead/dying tissues. This may occur on any plant part. It can be associated with an entire plant part, or a small part of a spot on a leaf.

A LESION is a spot (or maybe a speck) on a leaf, stalk, stem, or root of varying size, shape and colour. In my mind, spots are rounded, whereas specks are ragged/jagged, but that might be just how I picture them. A lesion might have a different colour in the centre than that of the outer edge or surrounding tissues. It might be round, oval, blocky, delineated by veins (or not). A lesion might have concentric rings or might be water-soaked, depending on the pathogen. There might be areas with spores or spore-bearing structures within the bounds of the lesion, resulting in a speckled or spotty appearance. On grassy plants, lesions may be more striped than spotted. Leaves may be discoloured, without noticeable spots, perhaps from the leaf margins inward, or with the parts or the entire leaf changing colour. Scorching can manifest as portions of the leaf turning red or brown rapidly, or tissues (leaves, twigs, shoots) might suddenly develop a burnt appearance, with the leaves remaining attached.

BLIGHT is a term that describes an accelerated tissue death, which may or may not start as a lesion, with leaf discolouration. The term “blight” can fit many different diseases; therefore, it is important to follow up with additional details (see above).

DIEBACK refers to when an entire shoot or branch of a plant suddenly and completely dies, often sagging and wilting as it dies.

CANKERS are a type of aggressive lesion on a supporting stem/stalk/trunk/branch. Cankers may be sunken and may have oozes, fruiting structures, and/or a watery appearance. They can be places where pathogen hang out for a time, until they are ready to dial things up a bit.

Any location where tissues start to break down or degrade en masse are referred to as ROTS. Rots may be watery, smelly, oozy, dry, crumbly, necrotic, or colourful. They can be confined to a single area or tissue type on the plant (e.g., vascular tissues), or they can take up a lot of space. Rots are “fun”, in that where one starts, other organisms jump in and take over. Some rots may have visible fungal bodies or fungal mycelium, whereas others are less obvious.

In situations where plants collapse, there are specific descriptive symptoms. Young plants (seedlings) that suddenly die of or collapse is referred to as DAMPING OFF, whereas larger plants that have all or a part of the plant wilting is different (dieback, wilts). WILTS (or wilting) can be diagnostic, in that some are specific to one side of the plant, whereas others are more general. Take a look at the colours of the vascular tissues (by cutting), as some diseases cause characteristic vascular discolouration.

COLOUR changes in plants can be confusing, since, while colour change is a signal that the plant or tissue is stressed by something, a single colour isn’t necessarily a perfect indicator of the cause. Chlorosis, or yellowing, can be localized (typically around a lesion or point of attack), or it may be associated with certain areas or tissues (leaf blade tissues, but not veins). You might also observe purpling or reddening, in different shades, which can be associated with nutrient deficiencies or perhaps frost damage. Marginal (leaf margin) colour change (yellow or brown) might be associated with salt or herbicide damage. MOTTLING or mosaics are often symptoms of some sort of virus or strange stress.

Plant DEFORMITIES also come in all different variations. STUNTING, or a reduction in size from normal, may occur at different stages, although it is mainly observed as the plant falls behind schedule, as it were. Leaves and other plant parts may CURL, in different directions (downward, upward, tip to base, etc.). This happens as tissues are commandeered by a pathogen or when plant tissues react to a change in hormone levels (or hormone mimics, a.k.a. herbicides).

SCABS or SCURFS are generally associated with tubers or roots, and represent fungal growth on the surface (e.g., sclerotia of Rhizoctonia), or the colonization of the skin cells by a pathogen.

Signs

Signs are different from symptoms, as they represent the physical observance or evidence of the presence of a pest. While we most often associate signs with things like insect pests, pathogens can also signal their presence as signs. As mentioned before, fungi have a vegetative stage, called mycelium, which is often observed as the pathogen spreads over the surface of the host. The presence of mycelium is sometimes commonly referred to as MILDEW. For some fungi, this is the main or most visible sign. When the term “MOULD/MOLD” is used, this typically describes a fungal pathogen with active colony that has some mycelium, but also spores. The observation of fruiting bodies or spore masses (e.g., something like Botrytis Grey Mold) should be noted and described (colour, quantity, associated tissue changes, etc.).

If “MUSHROOMS” are visible, their shape, size, colour, etc. can tell you about which organism is producing them. PYCNIDIA are often observed in the centres of lesions as visible little black specks, similar to flakes of pepper. Some fungi produce PUSTULES or raised/erupted forms. Characteristic pustules are produced by fungi such as rusts (which are conveniently coloured orange (for the most part)). The presence of bulging fungal bodies, such as those produced by smuts, is obvious. Some pathogens hijack the plant, causing massive cellular growth, resulting in GALLS, such as is the case with Black Knot of Prunus.

Some fungi produce survival structures, such as SCLEROTIA. These are compressed/hardened mycelial masses resembling rodent droppings. These can be found in rotting areas on the plant or on the surface of some fungal colonies.

The presence of nematodes may be confirmed by the presence of CYSTS (the swollen bodies of the mothers) on roots, or the nematodes themselves might be observed under a microscope.

Conclusions

There are many things to look for. Look closely, look often, and be descriptive. Please.