Plants are even more susceptible to diseases and pests than fish because there are many more vectors for infection. Spores are easily carried by the wind or by insects, while fish generally must be infected through bird or animal waste or contaminated water supplies. There are thousands of potential diseases and pests that can strike an aquaponics system, but the most common and devastating varieties are covered below.

Pythium: An aggressive fungus that is responsible for rot and stem rot in maturing plants and damping off in seedlings. It’s particularly aggressive towards seedlings and transplants in which it causes part of the stem to weaken, turn dark and collapse until the plant falls over. Mature plants will wilt suddenly and reveal rot among the roots or on the main stem. Pythium can only attack weak and stressed plants, so the best prevention and treatment is improving growing conditions. Proper aeration is the best tool to prevent it from causing root rot in maturing plants.

Early and Late Blights: While lettuce and other greens are relatively safe from this threat, both diseases can wipe out herbs and fruiting plants right before they’re ready to sell, especially tomatoes. Early blight cause rotten spots on the stem, yellow lesions on the leaves with brown circles in the center, and lumpy cankers at stem junctions. Late blight causes brown or gray leaf lesions as well but without the yellow color with no stem symptoms. Both are best treated by removing all infected plant material and spraying fungicides on any healthy plants before the infection can spread.

Powdery mildew: The most common fungal disease of crops in general is powdery mildew, and this remains true for aquaponics systems due to the relatively high humidity around the plants. High humidity encourages the fungi to flourish, spreading a white or hazy looking film over the leaves and stems of plants. The leaves affected by mildew eventually wilt and die back, starving the plant by interrupting its photosynthesis process. Your plants may look like someone has coated them with powdered sugar, which will usually start as just a few small circular spots of fuzzy white growth. There are dozens of treatments available, but most of them are not safe for fish. Skim milk sprays have been proven to treat and prevent powdery mildew in traditional agricultural settings and should work fine for aquaponics systems. Another option is to mix water with a pH level of at least 9.0 with 30% hydrogen peroxide at a rate of 3 mL of hydrogen peroxide per liter of water. This won’t damage the crops or fish while killing off the fungus.

Aphids: These pests are the most common insects to attack aquaponics systems, especially if they’re contained inside greenhouses. The enclosed space protects them from natural predators and encourages them to breed in huge numbers, which can cause serious damage. Thankfully, aphids are easily washed off with the right spraying equipment to knock them loose without damaging the leaves. There’s no need for specific pesticides just to get rid of an aphid population, just some thorough spraying a few times a day until they’re gone.

Fungus gnats: These little gnats are often found in homes with houseplants, as they move in anywhere damp soil is available for laying their eggs. While fungus gnat larvae do eat plant roots, they rarely cause any damage this way. The true danger of a fungus gnat infestation is due to their ability to spread Pythium and cause extensive damping off and rot. Fungus gnats can be caught with traps and managed by lowering ambient air humidity levels with proper ventilation and air exchange.

Slugs: Any wet and relatively warm environment will attract slugs, but aquaponics systems are even more attractive to them, thanks to the steady food supply. Hand removal is usually the only option available for aquaponics because traps like dishes of beer and piles of cornmeal rarely attract enough of them to dent the population when there’s a steady supply of food. Spreading thick and wide layers of diatomaceous earth powder around the edges of the troughs or tanks is the most reliable way to create a barrier to stop them from reaching the plants. This powder is all natural and won’t harm your fish, but it is made of very sharp particles that hurt the slugs’ soft bodies if they try to cross.