Article written by: Jason Danaher Ph.D.
Humans are not the only organisms that find lettuce, basil and cucumber a tasty treat and despite your best efforts your crop can become affected by disease or pests.The best form of defense against plant disease and pests is the farmer’s shadow, meaning the farmer needs to be actively looking for the presence and signs of disease and pests in and around their production area. "Scouting for disease and pests" in aquaponics means regularly inspecting your hydroponic plants throughout the growing cycle to identify any potential disease or pest early on by visually examining the plant’s leaves, stems, roots, and/or fruits. An effective treatment plan can be put into place once the disease or pest is properly identified. Below is a list of several common plant diseases and pests you may encounter growing lettuce, basil, and/or cucumbers in your aquaponic system.
Lettuce (Lactuca sativa)
Common Diseases of Lettuce
Bacterial Leaf Spot: This bacterial disease is caused by Xanthomonas spp. and results in brown/black water-soaked spots on the foliage which eventually makes the foliage turn necrotic and yellow. It thrives under wet and cool weather conditions.
Damping Off: Damping off is caused by a fungus that is active in environments with high humidity, growing media that is too wet, and air temperature above 68 degrees fahrenheit. There are several fungi which may cause damping off including Pythium, Rhizoctonia, Theleviaopsis, Fusarium spp. This disease affects seeds and seedlings in cool conditions with high humidity and moisture. Infected seeds will not sprout. Infected seedlings can have brown thin stems at their base and may be accompanied with a fluffy white cobweb-like growth surrounding the area where the stem protrudes out of the growing substrate. The seedlings and transplants will quickly die.
Downy Mildew: Downy mildew is a fungal disease caused by a variety of fungi including Peronospora, Bremia, Plasmopara, and Basidiophora spp. that attacks the leaves, stems, and flowers. It may appear on the undersides of leaves as gray fuzz, but on the topside of leaves it is often observed as a yellowish, patchworked pattern.
Fusarium Wilt: This fungal disease is caused by the pathogen Fusarium spp. causing yellowing and stunting of older plants. In younger plants it also causes yellowing, stunting and death. The plant will exhibit signs of frequent wilting before the lower leaves become desiccated and turn yellow.
Powdery Mildew: This fungus disease occurs on the top of the leaves in humid weather conditions. The leaves appear to have a whitish or greyish surface and may curl.
Common Pests of Lettuce
Aphids: These insects can come in a variety of colors (green, red, black or peach) and have piercing/sucking mouthparts to extract sap from the underside of leaves resulting in desiccated and dead leaf tissue. Waste expelled by aphids drip onto lower plant leaves resulting in a sticky, black residue known as honeydew and attracts ants.
Cabbage Looper: These caterpillars are green with a white stripe on either side and measure about 1.0 to 1.5 inches in length. They often tunnel through the heads of lettuce as they consume the leaf tissue.
Flea Beetles: These small insects are less than 1/4-inch in length and can come in a variety of colors (black, bronze, bluish, brown, or metallic gray). These small hopping beetles feed on plant foliage.
Leafminers: The maggots bore just under the leaf surface causing crooked, irregular, snakelike markings often resulting in disfigured foliage, but not plant death, before the maggots transform into adult flies.
Slugs: These pests are a shell-less terrestrial mollusks. They often leave a slime trail and prefer damp weather environments. Slugs can consume the entire leaf and are usually active in the evenings.
Basil (Ocimum basilicum)
Common Diseases of Basil
Bacterial Leaf Spot: This bacterial disease is caused by Xanthomonas spp. and results in brown/black water-soaked spots on the foliage which eventually makes the foliage turn necrotic and yellow. It thrives under wet and cool weather conditions.
Botrytis Blight: Also called gray mold, this causes a brown to gray fungus on plant leaves and stems. Diseased leaves die and fall off. If the infection is severe on the main stem the plant may die. The condition thrives in high humidity and cooler temperatures.
Downy Mildew: Downy mildew is a fungal disease caused by a variety of fungi including Peronospora, Bremia, Plasmopara, and Basidiophora spp. that attacks the leaves, stems, and flowers. It may appear on the undersides of leaves as gray fuzz, but on the topside of leaves it is often observed as a yellowish, patchworked pattern.
Fusarium Wilt: This fungal disease is caused by the pathogen Fusarium spp. causing yellowing and stunting of older plants. In younger plants it also causes yellowing, stunting and death. The plant will exhibit signs of frequent wilting before the lower leaves become desiccated and turn yellow.
Common Pests of Basil
Aphids: These insects can come in a variety of colors (green, red, black or peach) and have piercing/sucking mouthparts to extract sap from the underside of leaves resulting in desiccated and dead leaf tissue. Waste expelled by aphids drips onto lower plant leaves resulting in a sticky, black residue known as honeydew and attracts ants.
Japanese Beetles: This beetle measures approximately 0.6-inches in length and its body is generally metallic green with copper-brown wing covers. It has five distinctive patches of white spots on each side adjacent to the wing covers. The Japanese Beetle has a voracious appetite and often damages the tender leaves and new growth of the basil plant, leaving a lacy design of plant tissue on your plant.
Slugs: These pests leave large holes in the foliage or eat leaves entirely. They leave a slime trail, feed at night and are mostly a problem in damp weather.
Cucumbers (Cucumis sativus)
Common Diseases of Cucumbers
Damping Off: There are several fungi which may cause damping off including Pythium, Rhizoctonia, Theleviaopsis, Fusarium spp. This disease affects seeds and seedlings in cool conditions with high humidity and moisture. Infected seeds will not sprout. Infected seedlings can have brown thin stems at their base and may be accompanied with a fluffy white cobweb-like growth surrounding the area where the stem protrudes out of the growing substrate. The seedlings and transplants will quickly die.
Alternaria Leaf Spot: Alternaria cucumerina is a fungal disease that can affect cucurbit crops but is most common on melons and watermelons in areas with high amounts of rainfall, high humidity, and warm temperatures. Alternaria cucumerina often appears on the top of leaf surfaces as small brown, reddish spots with a yellow halo. Infected leaves can turn a brown color, wither and die shortly afterwards. Although fruits are not directly affected by Alternaria Leaf Spot, the withering of leaves and later leaf loss results in fruits suffering from sunscald.
Anthracnose: Anthracnose is caused by the fungus Colletotrichum orbiculare and proliferates during times of high humidity and warm temperatures. It is initially observed as irregular brown spots on the leaf surface. The fungus can also quickly spread to the ripening fruit and develop black, sunken depressions that will quickly enlarge and destroy the fruit.
Powdery Mildew: This fungus disease is common with cucurbits. It will grow and quickly spread on the top of leaves, stems, and petioles in humid weather conditions and appear as a white or gray powder on the plant’s surface.
Common Pests of Cucumbers
Aphids: These insects can come in a variety of colors (green, red, black or peach) and have piercing/sucking mouthparts to extract sap from the underside of leaves resulting in desiccated and dead leaf tissue. Waste expelled by aphids drips onto lower plant leaves resulting in a sticky, black residue known as honeydew and attracts ants.
Whiteflies: Whiteflies are tiny, flying insects with piercing/sucking mouthparts to extract sape from the underside of leaves resulting in desiccated and dead leaf tissue. Typically, whiteflies are white or pale yellow in color and adults are generally 0.03 to 0.08-inches in size. Whiteflies may become abundant on vegetable crops during warm, dry weather.
Cucumber Beetles: Striped or spotted cucumber beetles can damage cucumber plants in a variety of ways. Beetle larvae can bore through the roots of plants while the adults, approximately ½-inch in size or smaller, will feed on the stems, leaves, and fruits with their chewing mouthpart.
Squash Bugs: Squash bugs will feed on leaves and fruit with their piercing/sucking mouthparts to extract sap. Adults are 5/8-inch in size or smaller and have a gray or brown body color. Adult squash bugs give off a foul odor when crushed.
Spider mites: Mites are small arachnids (eight-legged arthropods) about the size of a grain of pepper (0.01 to 0.03-inches). These arachnids can come in a variety of colors (red, black, brown or yellow). Mites have piercing/sucking mouthparts to extract plant sap from the leaves and stems often desiccating the foliage and resulting in a yellow color. While individual mites are difficult to see with the naked eye, they often leave a visible webbing on the underside of the leaf. Mites quickly reproduce and thrive in dry conditions.
Cucumber Worm: These caterpillars are commonly known as pickle worms and can reach up to ¾-inch in size. These small green caterpillars cause significant damage to cucumber plants by tunneling through stems and bore through the sides of fruit, making it inedible and/or unmarketable. When they reach maturity the caterpillar turns into a small brownish-yellow moth.
Conclusion
Lettuce, basil and cucumbers are commonly grown in aquaponic systems and the successful production of each requires preventing plant disease and pests from threatening the production system. Carefully look at your plants for potential signs of disease and/or pests damage. Examine leaves, stems, near flowers, new growth, and even developing fruit. Early detection of problems, correctly identifying pests/diseases, assessing severity, planning treatment, and later evaluating treatment efficiency should be objectives of your facility management. Consider Integrated Pest Management (IPM) strategies for your operation that use a variety of control measures, including mechanical, cultural, biological, and chemical options to mitigate pressure from plant disease and pests.
Contact your local county Cooperative Extension Service for a list of IPM techniques including a list of the Organic Materials Review Institute (OMRI) fungicide and insecticide recommendations for integrated systems like aquaponics. Also, inquire with extension staff about disease resistant plant varieties that are known to grow well in your area and the type of hydroponic technology used in your aquaponic system. Feel free to call Aquatic Equipment & Design, Inc. staff at 407-995-6490 for additional information and tips on preventing plant disease and pests in your aquaponic system.