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Table 10: Role of nutrition in plant disease resistance.
| Nutrient | Effect |
|---|
| Nitrogen | Overfertilization 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. |
|---|
| Phosphorus | Improves nutrient balances and accelerates maturity of the plants. |
|---|
| Potassium | Accelerates wound healing and reduces the effect of frost damage. Delays maturity and senescence of plants. |
|---|
| Calcium | Reduces the severity of some root and stem fungal diseases. Affects the cell wall composition in plants that resist fungal penetrations. |
|---|
| Silicon | Helps plants produce specific defense reactions, including the release 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.
FIGURE 22a
Bacterial canker in leaf.
Bacterial canker: The bacteria that cause 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).
FIGURE 22b, c
Grey mold.
FIGURE 22d
Powdery and downy mildew on grape leaf.