From the plant's perspective, brown or crispy leaves are a direct signal of water stress. This condition arises from an imbalance between water uptake by the roots and water loss through transpiration from the leaves. When the roots cannot supply enough water to the leaves, the plant's cells lose turgor pressure—the internal water pressure that keeps them rigid. The leaf tissue, particularly at the edges and tips which are farthest from the vascular system, begins to desiccate and die, resulting in the brown, crispy texture you observe. This can be caused by two opposing environmental factors: either insufficient water in the soil (underwatering) or damage to the root system that prevents water absorption, which can ironically be caused by overwatering.
While it may seem counterintuitive, overwatering is a frequent cause of symptoms that appear to be drought stress. Chrysanthemum roots require oxygen to respire and function properly. When the soil is constantly saturated, air pockets are filled with water, leading to oxygen deprivation (anoxia). In this oxygen-poor environment, the delicate root hairs responsible for water uptake begin to suffocate and die. Once the root system is compromised, it can no longer effectively draw water from the soil, no matter how wet it is. Consequently, the leaves exhibit the same browning and crisping as an underwatered plant because, physiologically, the plant is experiencing a drought from its own damaged roots.
Environmental conditions can dramatically increase the rate of transpiration, placing excessive demand on the root system. Intense sunlight and high temperatures cause the plant's stomata (pores on the leaves) to open to cool the tissue through evaporation, which accelerates water loss. Similarly, windy conditions strip away the humid boundary layer of air around the leaves, further speeding up transpiration. If the roots cannot keep pace with this accelerated water loss, the leaves will scorch and turn brown. Low humidity creates the same effect, pulling moisture from the leaf tissues into the drier surrounding air.
The availability of nutrients in the soil directly impacts the plant's physiological functions. A deficiency in certain nutrients, particularly potassium (K), can lead to leaf scorch. Potassium plays a vital role in regulating the opening and closing of stomata and maintaining internal water pressure. Without adequate potassium, the plant cannot manage its water balance efficiently, making it more susceptible to stress. Furthermore, an excessive accumulation of soluble salts in the soil, often from over-fertilization, creates a high osmotic pressure around the roots. This makes it harder for the roots to absorb water, effectively creating a physiological drought even when the soil is moist.
Insect pests such as spider mites or thrips feed directly on leaf cells, piercing the tissue and sucking out the chlorophyll and fluids. This feeding damage kills small clusters of cells, which initially appear as speckles or stippling before merging into larger brown, crispy patches. Fungal pathogens, like those causing leaf spot diseases, infect the leaf tissue, causing localized cell death that manifests as brown or black spots with sometimes a yellow halo. As the infection progresses, these spots can enlarge and coalesce, causing large sections of the leaf to die and become crispy. These biotic stresses weaken the plant and disrupt its normal water and nutrient flow.
