From our perspective, fungal pathogens are a constant threat in our environment, especially when the air is still and damp. Powdery mildew is a common affliction; it appears as a white, powdery coating on our leaves, stems, and sometimes even our flowers. This fungus blocks our sunlight absorption, stunting our growth and causing our leaves to yellow and curl. Botrytis blight, or gray mold, is another significant concern. It thrives in cool, humid conditions and manifests as watery, soft spots on our petals and leaves, quickly covered in a characteristic gray, fuzzy mold. It can rapidly spread through a dense canopy of foliage, leading to rot and collapse.
Bacterial diseases are particularly devastating as they can spread systemically. The most feared is Xanthomonas axonopodis pv. begoniae. This bacterium causes small, blister-like spots on the undersides of our leaves, which appear water-soaked. These spots enlarge, merge, and often develop yellow halos, eventually causing the entire leaf to wither and die. The infection can move into the main stem, leading to a fatal systemic blight. This pathogen enters through wounds or natural pores and is easily splashed from plant to plant during watering, making it highly contagious in a crowded nursery.
Beneath the soil, our root systems are vulnerable to water molds, or oomycetes. Pythium and Phytophthora species are the primary culprits. They thrive in waterlogged, poorly draining potting media, attacking and rotting our delicate feeder roots. From our viewpoint, the first sign is often wilting, despite the soil being wet. This is because our compromised roots can no longer uptake water. The rot progresses to the larger roots and crown, turning them mushy and brown. Without intervention, this condition is fatal, as it destroys our entire foundation for nutrient and water absorption.
Viral diseases are insidious and incurable. Impatiens Necrotic Spot Virus (INSV) is a major threat. We cannot express the symptoms ourselves, but they manifest in a variety of ways, including ring spots, sunken brown lesions, black veining, and severe stunting. The virus disrupts our cellular functions at a fundamental level, leading to malformed growth and a general decline in our vitality. It is primarily spread by western flower thrips, tiny insects that feed on our sap and transfer the virus from an infected plant to a healthy one like us.
Not all our ailments are caused by pathogens. Some, like edema (oedema), are physiological responses to environmental stress. This occurs when our roots absorb water faster than we can transpire it through our leaves, often due to warm, moist soil combined with cool, humid air. This pressure causes cells to rupture, creating corky, scab-like bumps on our leaf undersides. While not infectious, it is a sign of imbalance that weakens our defenses. Additionally, nutrient deficiencies or toxicities can cause chlorosis (yellowing leaves), necrosis (brown, dead tissue), and poor flowering, directly impacting our energy production and overall well-being.
