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Plant disease problems can be difficult and time consuming to treat. Preventing issues from arising is the first step in proper plant care. Many foliar plant diseases are present during conditions of high temperature and humidity. Providing proper ventilation and reducing humidity will prevent conditions that allow mold and disease to spread to other plants.
Plant nutrition plays a direct role in disease resistance in plants (Agrios 2005). Providing the correct balance of nutrients is important not only for growth but also to decrease susceptibility and increase recovery from certain plant disease. Table 10 describes the role of certain nutrients for prevention of plant disease. Below are common plant diseases in aquaponic systems.
Table 10: Role of nutrition in plant disease resistance. NutrientEffectNitrogenOverfertilization makes more succulent tissues that are more prone to fungal attack. Nitrogen starvation results in stunted plant that are more prone to attack from opportunistic micro-organisms.PhosphorusImproves nutrient balances and accelerates maturity of the plants.PotassiumAccelerates wound healing and reduce the effect of frost damage. Delays maturity and senescence of plants.CalciumReduces the severity of some root and stem fungal diseases. Affect the cell wall composition in plants that resist fungal penetrations.SiliconHelps plants produce specific defense reactions, including the releases of phenolic compounds against pathogens.
Bacterial canker: The bacteria that causes bacterial canker, Pseudomonas syringae, enters the plant through existing wounds caused by pruning, harvesting, or injury. Signs of bacterial canker include marginal browning or necrosis on leaves, elongated tan regions or splitting of the stem, and/or small white spots on the fruit (Figure 22a). The most common cause is unsanitary growing condition or harvesting tools.
Grey mold: Caused by the pathogenic fungus, Botrytis cinerea, grey mold can be found almost anywhere plants are grown. Prevalent during damp, cool weather, grey mold can spread quickly through the crop, affecting stems, leaves, and fruits. Leaves may have brown lesions that spread over the entire surface, causing the leaf to wilt (Figure 22b). If not controlled, spores will spread to flowers and fruits, where fuzzy, grey growth will appear (Figure 22c). Improving ventilation with fans and air flow within the plant structure through pruning are preventative measures. In addition, removing fallen or diseased plants and avoiding injury to trellised plants is critical in preventing grey mold.
Powdery and downy mildew: These two types of mildew affect nearly all vegetable crops. Primarily affecting the leaves of the plant, they are more prevalent in humid conditions. Powdery mildew is circular and white in appearance and can appear anywhere on the leaf surface. The leaf may yellow if the fungus has been present for a long time. A downy mildew spot is angular and grey in appearance and the fungus is limited by the leaf vein. Leaves may appear yellow before the presence of the fungus is evident (Figure 22d).
Pythium: The causative agent for root rot in plants, Pythium sp. are found naturally in the culture environment and impact a wide variety of plants. Symptoms include brown, rotting roots that slough off easily when disturbed (Figure 22e). Plants may appear stunted or nutrient-deficient. Different species of Pythium are prevalent at specific temperatures; however, in aquaponics they commonly appear at water temperatures above 78°F and conditions with high organic solids. Controlling temperature and implementing effective solids removal will limit Pythium sp. in an aquaponics system.
Source: Janelle Hager, Leigh Ann Bright, Josh Dusci, James Tidwell. 2021. Kentucky State University. Aquaponics Production Manual: A Practical Handbook for Growers.