Is It A Disease or A Deficiency?

Picture this, you’re walking around your garden and you notice something is wrong. Some of your plants don’t look quite right. The leaves are discolored and curling. You know you’ve been watering regularly and they are getting the right amount of sun. You don’t see any signs of pests attacking the plants, so what’s going on? Is it a disease or a deficiency? It can be hard to tell the difference, but here are three questions to help you diagnose the problem.

What Color Are the Leaves?

Look closely at the leaves of the affected plants. Are they turning yellow or some other color? Or turning brown at the tips and curling their leaves? Are some of the leaves turning all brown or black and dying or falling off? Different diseases or deficiencies can have tell tale signs that can help you diagnose the specific problem.

For example, a nitrogen deficiency can appear as leaves turning yellow. Sometimes it’s the whole leaf, or just the area in between the leaf veins. The leaf might be yellow but otherwise healthy for a while. But eventually it will turn brown and die as the plant runs out of food. On the other hand, a phosphorus deficiency can see the leaves “bronzing” to a purple or red color.

However, if the leaf gets a chalky grey coating over the top, you may be dealing with a mold, like powdery mildew. Other diseases can cause leaves to turn brown or black and die, but often more quickly than a nutrient problem. So, the first question in determining if it is a disease or a deficiency is to identify how the leaf color is changing.

Are There Spots?

Next look for any spots on the leaves, stems, or fruit. First, you need to make sure that birds or insects didn’t make the spots. Then check that parts of the plants aren’t rubbing against their supports or other plants. The sun can sometimes make scald spots on the tops of leaves when water droplets act as magnifying glasses on a sunny day. Also check to make sure the spots are not dirt, mulch, or grass clippings splashed up on the plant by rain or your mower.

There are several different plant diseases that cause spots. Some of them even have “spot” in the name, like Bacterial Speck, Bacterial Leaf Spot, or Dark Leaf Spot. If you’re seeing dark spots on your plants’ leaves, it may be a key sign of a disease. Also check for spots on the fruit and stem, and monitor if the spots are getting bigger or more numerous.

However, sometimes a change in leaf color can start at the tips of the leaves, so it may appear to be a spot on the leaf, but then eventually the whole leaf changes. This may be more typical of a nutrient deficiency rather than a disease. Blossom end rot is another deficiency that starts out as a spot on the end of the fruit and can cause the fruit to rot eventually, even though it’s not caused by a bacteria, virus, or fungus.

Is it Spreading?

When you first notice a problem with your plants, it’s logical for you to check nearby plants for the same problem. If you see problems elsewhere, try to identify how the problem appears to be spreading. Is it affecting the whole plant, or just a few branches or fruits? Are the affected areas on different plants close to or touching each other? Does it appear to be moving outward from one location?

For example, if you see a whole bed of a particular plant with discolored leaves, that may indicate a problem with the soil. But if you see only the bottom most leaves turning yellow with brown spots, that may indicate a disease pathogen has transferred up from the ground somehow.

If one end of your bed has affected plants, but the other seems fine, you could have different soil nutrition, or you could have a spreading disease. You may not be able to tell until you monitor the progress (or fully diagnose the problem). If the plants in the middle are affected from one side, and you haven’t moved any soil around, or watered or fertilized unevenly, it’s likely a spreading disease.

Next Steps

Once you’ve used the three questions above to tell if it is a disease or a deficiency, you can start planning your response. As they say, “knowing is half the battle”. You will treat a nutrient deficiency very differently from a disease. For a deficiency you could get your soil tested and add nutrients to the soil with compost, fertilizer, or specific soil additives. For a disease you may need to spray with anti-bacterial or anti-fungal chemicals. Or remove and dispose of the affected plants to stop the disease from spreading. Get the right diagnosis so you can figure out the right solution!

Did you find a plant disease in your garden? Don’t despair! Check out my blog post on how to Save Your Plants from Disease for all my advice on rescuing your plants and keeping plant diseases from attacking your garden in the future.

Need somewhere to document what you find in your garden? How about my Ultimate Garden Workbook? It has everything you need to document your garden journey and keep you on track.