Diagnosis for Dummies: A Reference for the Rest of Us

Matt LeBannister

Welcome to the world of indoor gardening. It is an exciting one, but when you move a garden indoors you take on Mother Nature’s role and become responsible for all aspects of plant development. It may seem onerous at times, but it doesn’t have to be. Any seasoned gardener will tell you tales of the myriad problems he or she has faced over the years with regard to plant health, but some basics are discussed here that will save you time and trouble.

Assuming you have an ambient temperature in your growing space between 65ºF and 85ºF (~18ºC and 29ºC) and humidity in the range of 40 per cent, most plant problems fall into two categories. Usually you will discover a nutrient problem such as toxicity, deficiency, or lockout. Otherwise there is likely a pest or disease present, for example, mites, gnats, moulds, mildews, and wilts. By learning the early signs of plant problems and by using the process of elimination you can quickly make a diagnosis.

Nutrient Problems
The first step that every gardener should take once a nutrient disorder is established is to flush the plant well with pH-balanced water. This flushing can do no harm and will remove any excess salts from the medium and the plant itself. Next, make any necessary changes to the feeding schedule to prevent future problems. A helpful reference is available online at www.hydroponics.com/info/genproblems.htm.

Below is a quick guide to help you determine whether your plants require more nutrition or have been given too much. These are the most common nutrient disorders that are encountered by the average indoor grower.

Element	Deficiency Symptoms	Toxicity Symptoms
Nitrogen	Weak, stunted plants; yellowing lower leaves, starting at the tip, and moving from the old to the new growth as the deficiency progresses	Weak, stunted plants; yellowing lower leaves, starting at the tip, and moving from the old to the new growth as the deficiency progresses
Phosphorus	Stunted and very slow growth; dark bluish-green leaves, often with spots; when blotches on leaves reach stem, leaves turn bronze-purple, contort, and drop; often caused by pH under 5.8 or over 7.0	Excessive levels of phosphorous show up as a micronutrient deficiency, usually a zinc deficiency
Potassium	Plants appear healthy, with dark green leaves; leaf margins turn grey, then progress to a rust colour, curl up, and dry; leaves curl up, rot sets in, and older leaves drop	Locks out micronutrients and causes a micronutrient deficiency; leaf burn and wilting
Calcium	Younger leaves are immobile and their edges curl; plants will be stunted and have dark, crinkly leaves	Appears as magnesium deficiency because high levels of calcium cause soil and water to become alkaline.
Magnesium	Leaf veins stay green while the rest of the leaf turns yellow; brown spots appear and the plant dries out. To cure, foliar spray with water containing a pinch of magnesium sulphate.	Small necrotic spots in older leaves and smaller veins in older leaves may turn brown; in advanced stage the young leaves may be spotted
Iron	Tips of new leaves will become either pale or yellow and this will spread inward; leaf will likely turn blotchy from a lack of chlorophyll, eventually turning brown and drying out	Possible bronzing of leaves, with tiny brown spots

The solutions to these problems are relatively simple once they have been identified. For any toxicity problem, flush with pH balanced water or a clearing solution and correct your feeding schedule accordingly. A deficiency can be corrected by adding more of the nutrient that your plants are missing. Using a well balanced, two-part nutrient should avert this problem before it ever starts. Always check the pH of the nutrient solution. The pH of your nutrient solution is directly related to the uptake of the nutrient by your plants. Drift too far out of the plants’ optimal range and you will find nutrient discord common.

The use of humic acid, which chelates the micronutrients, and fulvic acid, which chelates the larger nutrients, will help prevent any deficiencies and almost always increase the yield and quality of your crop. There are many products on the market that contain these specialized ingredients. Look for the brand names containing the word “black” for humic acid and “gold” for fulvic acid.

Pests and Disease
Mites, gnats, moulds, and wilts can sneak their way into a grow room without leaving a trace as to how they got there. They may be introduced by infected plants, by air containing fungus and mould spore, or even on the sleeve of your shirt. With some due diligence they can be prevented, but if allowed to grow unchecked, extreme measures may be needed to eradicate the problem before it quickly eradicates your crop.

A few tips that will help greatly and apply to any of the problems are described below. First, cleanliness is particularly important and, indeed, next to godliness. Most pests prefer to consume organic waste matter, so keep your growing space free of debris. Remove dead leaves and all expired organic matter from the room, including the top of the media. Keep tools clean and wash them after each use to prevent spread of disease throughout your plants.

If you believe there is a possibility of an insect attack or mould or disease, quarantine the suspected plants immediately. It is always recommended to sacrifice the plants that are affected, to protect the healthy. You may find it necessary to surrender half of your crop, but with a quick diagnosis the other half can be saved.

Spider Mites
Identification: Spider mites are the most common pest found attacking indoor plants. They can also be one of the most devastating because each female can lay up to one hundred eggs. Spider mites have eight legs; can be yellow, white, brown, two-spotted or red; and appear as tiny specks on the undersides of leaves. Webbing is a sure sign that the infestation is in the advanced stage. Damage: Mites will suck the sap out of the leaves. This gives leaves a pocked appearance and causes yellowing because of failure to produce chlorophyll. Control: For light infestations, pine and neem oil are favourites for controlling mite populations; it is organic and safe to use until harvest. For heavier infestations a miticide may be necessary. Products with a pyrethrum base are favoured, but mites can adapt to any remedy quickly, so you need to switch remedies often. For very bad infestations the entire grow room will have to be cleaned out and disinfected with a five per cent bleach solution. Lower the room temperature and increase the humidity to deter the pest.

Fungus Gnats
Identification: Larvae grow 4–5 mm long and have translucent bodies with black heads. Adults are black, 2–4 mm long, and have wings. You will find them around the base of the plants, trying to hide in the growing medium. They thrive in a damp environment. Damage: Fungus gnats eat fine root hairs and scar larger roots, causing plants to be stunted. The larvae prefer to eat decaying organic matter. The root wounds can invite wilts. Control: The best solution is to use beneficial nematodes, a natural way of dealing with larvae and available at most indoor gardening stores. To control the adults, yellow or blue sticky cards can be very effective.

Thrips
Identification: Thrips are 1–1.5 mm long, are very fast moving, and come in a variety of dark colours as well as white and grey. They can also have stripes. They like to hide under leaves and prefer to jump to safety rather than fly. You will see the black specks of thrip feces. Damage: Thrips scrape leaf tissue to suck out the juices within. The damage looks a lot like that of mites. Control: Spray with pine or neem oil or a pine- and neem-based product. Blue sticky cards also work well for population control. A predatory mite is an excellent biological option.

Whiteflies
Identification: Whiteflies look like a small white moth, about 1 mm long, and will usually appear at the top of the least healthy plant first. Eggs can be found on the underside of leaves. Damage: Whiteflies feed in the same way as mites and thrips, so the symptoms of damage are the same. Control: Very similar to mite control. Hang yellow sticky cards just above the canopy. Insecticidal soap or pyrethrum-based aerosols are also effective. As far as biological means, the wasp Encarsia formosa is the most helpful predator.

Damping-Off
Identification: This fungal condition is found in growing media. It is caused by Pythium, Fusarium, or Botrytis fungi. Damage: It attacks seedlings and cuttings, causing them to rot where the stem meets the growing medium, making them eventually fall over. Control: Damping-off is aggravated by a combination of the following: fungi present in the growing medium, overwatering, and excessive humidity. Lower the humidity and exchange the air, and use sterile growing media. A weak concentration of H2O2 as a media soak, and a five per cent bleach solution on cutting tools is a good idea.

Powdery Mildew
Identification: Powdery mildew is limited to the upper surfaces of leaves. Small spots will form there and progress into a fine white powder that will almost cover leaves, new shoots, and buds. Plants can be infected for weeks before they show the first signs of the infection. Damage: Leaves will yellow and eventually the plant will die. Control: To prevent and control mildew, exchange air frequently and keep humidity in the 40 per cent range. A sulphur-based product will kill moulds, mildew, and fungi, but can leave a bad taste in the plant. Pine oil is organic and biodegradable and will control the problem without affecting taste.

Root Rot
Identification: This fungus causes healthy roots to go from white to a mushy brown. If it progresses, the base of the stem can turn brown. Damage: Plant growth will slow. If it is not stopped early enough, the plant will die. Control: Always make sure you use a sterile growing medium. Don’t overwater and make sure there is lots of available oxygen to the root zone.

Viruses
Identification: Viruses are the last problem a grower wants to face. They can be spread by insects, other plants, and by the human hand. Damage: Viruses usually destroy a plant’s fluid conduit system. The plant’s growth will be stunted and it will look sickly. Leaf spots usually appear. Control: Once your plants have a virus there is little that can be done. Dispose of any infected plants; you might be able to save the rest of the harvest. Always use fresh sterile media and disinfect tools and hands before working in your garden.

Now that you know about the more common health problems you could see in your indoor garden, take that knowledge and a keen eye back to your plants. Never forget that prevention is the key. The best plant problem is the one you never get!