From our perspective, fungal pathogens are a constant threat, exploiting weaknesses in our defenses, often exacerbated by environmental conditions. Powdery mildew (caused by *Podosphaera xanthii*) appears as a white, powdery coating on our leaves and stems. It hinders our photosynthesis, weakening us and deforming our new growth. It thrives in humid conditions with poor air circulation. Another pervasive foe is gray mold or Botrytis blight (*Botrytis cinerea*). This fungus causes a soft, brown rot on our flowers, leaves, and stems, quickly covering them in a fuzzy gray mold, especially in cool, wet weather. It often enters through wounds or senescing tissues. Furthermore, leaf spot diseases (e.g., *Septoria chrysanthemi*) manifest as dark brown or black spots on our foliage. These spots can coalesce, causing our leaves to yellow, wither, and drop prematurely, severely reducing our vitality.
Bacterial diseases are particularly devastating as they can spread internally through our vascular system. Bacterial leaf spot (*Pseudomonas cichorii*) begins as water-soaked lesions on our lower leaves that expand into dark brown or black spots, often leading to extensive leaf loss. More severe is Bacterial wilt (*Erwinia chrysanthemi*), which causes a rapid and fatal collapse. The bacteria multiply within our stems, blocking the xylem and preventing water transport. You will see us wilt suddenly, our stems become soft and rotten, and if cut, a slimy ooze may be present. This disease is highly contagious and can decimate entire plantings.
Viral diseases are insidious; they stunt our growth and distort our appearance without cure. They are primarily spread by sap-sucking insects like aphids. Chrysanthemum virus B (CVB) is a common culprit. It causes a range of symptoms including mosaic patterns of light and dark green on our leaves, leaf distortion, and reduced flower size and number. Another is Tomato spotted wilt virus (TSWV), which leads to ring spots, line patterns, and severe bronzing on our leaves, often stunting us severely and causing flower color breaks. These viruses sap our energy and compromise our overall aesthetic value and health.
Below the soil, microscopic roundworms called nematodes attack our root systems. The most significant are root-knot nematodes (*Meloidogyne* spp.). They infect our roots, causing the formation of characteristic galls or knots. This damage disrupts our ability to uptake water and nutrients efficiently. Above ground, this manifests as stunting, yellowing (chlorosis), and wilting, especially during the hottest part of the day, as our compromised roots cannot meet our water demands. We become generally unthrifty and more susceptible to other stressors.
