Chlorosis is the presence of yellow leaves on a squash plant, which signals that the plant is experiencing stress. Chlorosis occurs due to a lack of chlorophyll production, the pigment responsible for green coloration and photosynthesis. This discoloration is a symptom, indicating an imbalance in the plant’s environment or internal processes. Causes range from improper watering to nutrient deficiencies, pests, or disease. Observing the yellowing pattern—where it starts, how it spreads, and what other symptoms are present—is the first step toward accurate diagnosis and treatment.
Watering, Sunlight, and Temperature
Squash plants require consistent moisture and are sensitive to both under- and overwatering, which rapidly induces chlorosis. Overwatering saturates the soil, depleting oxygen and causing root rot, which impairs the plant’s ability to absorb water and nutrients. This results in wilted and yellowing leaves, often with the soil remaining visibly wet. Conversely, underwatering causes general plant stress, appearing as uniform yellowing and brittle texture in the older leaves as the plant conserves moisture. Gardeners should check the soil moisture a few inches down; water deeply if dry, or withhold water if soggy.
Environmental factors like light and temperature also play a role. Squash thrives in full sun, and insufficient light can cause the lower, older leaves to yellow and eventually drop as the plant prioritizes energy for newer growth. Temperatures outside the optimal range of 70 to 85 degrees Fahrenheit can also cause stress. Cold stress below 50 degrees Fahrenheit can temporarily stunt growth and cause general yellowing, while excessive heat can lead to sun scald.
Identifying Specific Nutrient Deficiencies
When environmental factors are ruled out, yellowing often points to a lack of essential nutrients, and the location of the chlorosis provides a diagnostic map. Nitrogen (N) deficiency is the most common, as squash plants are heavy feeders requiring high amounts of this nutrient. Since nitrogen is mobile, the plant moves it from older leaves to support newer growth, causing the lower, older leaves to yellow first, with the discoloration progressing upward. Remediation involves side dressing the plant with a nitrogen-rich fertilizer or compost.
Other nutrient deficiencies exhibit distinct patterns based on mobility within the plant. Iron (Fe) and Manganese (Mn) are immobile nutrients, meaning they cannot be relocated from old tissues to new ones. A deficiency in these micronutrients manifests as interveinal chlorosis—yellowing between the veins while the veins remain green—on the newest leaves and growing tips. Magnesium (Mg) is a mobile nutrient, but its deficiency also causes interveinal chlorosis, which appears first on the older leaves, sometimes giving them a marbled or mottled appearance.
Pests and Fungal Diseases
Biological agents can cause yellowing as a secondary symptom of tissue damage or systemic infection. Sap-sucking insects like squash bugs and aphids feed by piercing the leaf tissue and extracting plant fluids, leaving small yellow or white spots known as stippling. Squash bugs inject a toxin that causes localized yellowing that eventually spreads, leading to scorched leaves. Aphids often cluster on the undersides of leaves, and their feeding can result in distorted, yellow leaves.
Fungal diseases are a frequent cause of chlorosis, especially powdery mildew, which is identifiable by the white, powdery patches it forms on the leaves. These spots start as yellow areas that turn white and eventually cause the entire leaf to yellow, shrivel, and die. More serious systemic issues include viral and bacterial infections, such as Squash Mosaic Virus, which causes leaves to develop a mottled, irregular yellow and dark green mosaic pattern. If the yellowing is accompanied by rapid, irreversible wilting, it may indicate Bacterial Wilt, which requires the immediate removal and destruction of the infected plant to prevent further spread.