Plant Problems: Pest, Disease or Deficiency?

Plant Problems: Pest, Disease or Deficiency?

You often don't know you have a plant problem until you see its aftermath -- droopy stems, yellowing foliage, curled leaves or other ugly damage. By then you'll need to act fast to determine the cause and start treating it before the problem spreads to the rest of your hydroponic operation. You can begin with this quick guide to a dozen common hydroponic plant problems and smart, effective solutions for them.


  • Symptom: Tiny yellow spots and splotches of black mold on leaves.
  • Cause: Whiteflies are all too common in indoor hydroponic gardens. They suck sap from leaves, causing the yellow spots. They also secrete sticky "honeydew" that becomes a breeding ground for sooty mold.
  • Solution: Set up sticky traps to capture the flying pests. Spray affected hydroponic plants with insecticidal soap.


  • Symptom: Tender young leaves are curled and coated with a shiny, sticky substance.
  • Cause: Aphids are sap-sucking little insects that are frequently found in the center of new shoots or under leaves. The honeydew they secrete can attract ants and fungi into your hydroponic systems.
  • Solution: Treat areas where the pests have gathered with a neem-oil spray that disrupts their growth but does not impact plants. You may need to reapply two or three times to ensure that all of the pests are eliminated.


  • Symptom: Leaves have dry white streaks, as if the chlorophyll has been stripped from them. You may also see green-black specks on the leaves.
  • Cause: Thrips are another pest that sucks sap from plants and dehydrates them. The insects leave behind the white streaks and black excrement.
  • Solution: Target these hydroponic pests with insecticidal soap or use a neem-oil spray for persistent infestations.

Damping Off

  • Symptom: All seedlings seem healthy and then die suddenly.
  • Cause: Damping off is a fungal disease that infects sprouts right at the soil line. If your seedlings all fall over without warning, damping off is almost certainly the culprit.
  • Solution: Use soil-less seedling mix, not potting soil, for planting seeds or clones. Keep the mix damp, but not soggy. Sprinkle a little peat moss or corn meal around the base of seedlings to absorb excess water, where the fungi breed.

Powdery mildew

  • Symptom: White dust on the tops and bottoms of leaves, especially where they meet main stems.
  • Cause: Powdery mildew is a fungal disease that thrives in the humid condition of indoor gardens. It can block light from reaching leaves and slowly weaken the plant.
  • Solution: Air circulation -- even just from a small fan -- reduces humidity and keeps the fungi from getting established. Isolate affected plants -- the disease can spread quickly -- and spray them with a sulfur-based fungicide.

Root Rot

  • Symptom: Lower leaves turn yellow and drop. Plant may appear wilted and fail to perk up after watering. When you lift the plant from the pot, the roots are brown and mushy rather than white and firm.
  • Cause: Several types of fungi cause root rot, which spreads through the root system in a week to 10 days, eventually killing the plant. Warm, stagnant water—in poorly draining containers or systems with insufficient oxygen—is a breeding ground for these fungi.
  • Solution: Be sure that your plants’ roots are not fully immersed in standing water or nutrient solution for hours. If you’re growing in pots, check to see that they drain quickly after watering. For more active type hydro systems, such as flood-and-drain set-ups, increase the frequency and duration of time with the air pump on to raise the oxygen level in the fluid and prevent the fungi from colonizing it. Use only freshly mixed nutrient solution and keep it cool--ideally around 68 degrees F—because the oxygen concentration diminishes as the temperature rises. In any system, you can get even more protection from root rot by introducing beneficial bacteria (available in liquid form) that ward off the troublesome fungi. test

Heat Stress

  • Symptom: Top leaves are curled and dry, and may appeared burnt.
  • Cause: If the indoor grow lights are too close to the plant or the grow room is too warm, plants may suffer heat stress.
  • Solution: If your hand feels hot when you hold it at the top of the plants, the lights are too close. Raise them a few inches and wait a few days. If symptoms persist, raise the grow lights a bit higher.

Nitrogen Deficiency

  • Symptom: Leaves on the lower part of the plant start to turn yellow and then gradually all but the top leaves look pale.
  • Cause: Nitrogen is the key nutrient for healthy leaf growth and when it's in short supply only the top growth will be robust.
  • Solution: Feed your hydro plants with a nutrient formula that's high in N from its early growth period until the leaf canopy reaches full-size. Add an amino-acid supplement to your nutrient solution to increase plants' absorption of nitrogen.

Nitrogen Oversaturation

  • Symptom: Tall plants with weak stems and very dark green leaves that curl downward. Mature plants may be lush and green, but do not flower.
  • Cause: Excessive nitrogen can stimulate fast but not sustainable growth. It can also hold plants in their vegetative stage and hinder blooming.
  • Solution: Flush your hydroponic plants with plain water for a day or two when you see these symptoms. Then reintroduce nutrients at a low concentration and incrementally increase the amount to the recommended levels. To encourage blooming in mature plants, switch to a potassium-rich nutrient formula.

Phosphorus Deficiency

  • Symptom: Small, slow-growing plants that frequently drop leaves. Foliage may be tinted dark green, purple or blue.
  • Cause: Phosphorus plays a vital role in growing roots and converting sunlight into energy for growth. Insufficient P will lead to slow growth and leaves that lack bright green chlorophyll.
  • Solution: Add a bone meal supplement to your daily hydroponic fertilizing routine. It delivers a natural boost of phosphorus fast.

Potassium Deficiency

  • Symptom: Scorched tips and edges of leaves. Limp branches. Weak blooming of flowering indoor plants.
  • Cause: Potassium is the building block of sturdy plants and it's essential for plant reproduction (aka flowering). Potassium deficiency can look at first like heat or grow light stress, but the weak stems and limbs tell you that your plants need more K.
  • Solution: As plants mature, their need for nitrogen diminishes and the demand for potassium increases, especially for flowering plants. When your plants are ready to flower, switch to hydroponic nutrient formulas with a high ratio of potassium to nitrogen.

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