The sudden appearance of yellow leaves on your squash plant, known as chlorosis, signals that the plant is under stress. Chlorophyll, the green pigment responsible for capturing sunlight, is being lost faster than it can be produced, causing the foliage to fade from deep green to shades of yellow. This discoloration is a visible symptom, indicating a deficiency or environmental imbalance that must be diagnosed quickly. Pinpointing the exact cause requires careful observation of the yellowing pattern and the location on the plant.
Yellowing Caused by Nutrient Shortages
Squash plants are heavy feeders, and their rapid growth demands a steady supply of nutrients. A lack of mobile elements, which the plant can relocate from older leaves to new growth, typically causes yellowing to begin on the bottom, mature leaves first. Nitrogen deficiency is the most common example, causing the entire older leaf to turn a uniform pale yellow as the plant pulls this element to support new shoot development. Applying a high-nitrogen fertilizer or composted manure can correct this issue quickly.
A shortage of magnesium also presents on older leaves because it is mobile, but the pattern is distinctively different. Magnesium is a component of the chlorophyll molecule; when deficient, the tissue between the leaf veins turns yellow while the veins remain green, creating “interveinal chlorosis.” A foliar spray or soil drench of Epsom salts (magnesium sulfate) can be an effective remedy.
In contrast, deficiencies of immobile nutrients, such as iron, first affect the youngest leaves at the top of the plant. Iron is required for chlorophyll synthesis but cannot be moved from older tissues, so new leaves display the deficiency first. These young leaves show interveinal chlorosis, similar to magnesium deficiency, but appearing on new growth. Treating iron deficiency involves applying a chelated iron supplement to the soil, as this form is readily available for plant uptake.
Yellowing Caused by Too Much or Too Little Water
Water is the vehicle for nutrient transport in the plant, and any imbalance can mimic a nutrient deficiency. Both overwatering and underwatering cause leaves to yellow, but the symptoms offer contrasting clues for diagnosis. Overwatering is a common issue that causes yellowing, typically starting with the lower leaves.
When the soil remains perpetually soggy, the roots become deprived of oxygen, leading to root suffocation and rot. Damaged roots cannot absorb nutrients or water, causing the foliage to turn yellow and often wilt, even in saturated soil. To check, push your finger two inches into the soil; if it feels wet or muddy, reduce watering frequency and ensure the planting area has adequate drainage.
Underwatering or drought stress causes a different set of symptoms. When the plant is not receiving enough moisture, the leaves often become brittle and crispy, and the yellowing may be accompanied by brown, dry edges. The plant will wilt during the hottest part of the day as it struggles to maintain turgor pressure.
A thirsty plant may recover slightly overnight, which helps distinguish it from an overwatered plant whose wilting leaves feel soft and limp. The solution for drought stress involves providing consistent, deep watering, ensuring the plant receives about one inch of water per week during hot, dry periods. Applying organic mulch around the base of the plant helps regulate soil temperature and conserve moisture.
Yellowing Caused by Pests and Illness
Biological agents like insects and diseases can cause yellowing, often presenting with specific patterns that distinguish them from cultural problems. Squash bugs and spider mites are common pests that feed by piercing the leaf tissue and sucking out the plant’s sap. This feeding causes distinct damage known as stippling, which appears as small, pale yellow or white speckles.
As the pests continue to feed, these speckled areas merge, causing entire sections of the leaf to yellow, leading to a scorched or tattered appearance. Inspecting the undersides of the leaves for the physical presence of mites, their fine webbing, or the bronze-colored egg clusters of the squash bug confirms an infestation. Treating these pests involves manual removal or applying insecticidal soap to the leaf surfaces, particularly the undersides.
Viral diseases, such as Squash Mosaic Virus, cause a distinct, irregular yellowing pattern. Infected leaves display a mosaic of light and dark green areas, often accompanied by misshapen, puckered, or stunted new growth. Since there is no cure, the infected plant must be removed immediately and destroyed to prevent aphids and other insect vectors from spreading the virus to healthy plants.
Fungal diseases like powdery mildew and downy mildew also contribute to foliage discoloration. Powdery mildew appears as white, powdery spots, usually on the upper side of older leaves, which eventually turn yellow and die. Downy mildew causes yellow or tan angular spots confined by the leaf veins, sometimes with a grayish, downy growth visible on the leaf underside. Managing these diseases involves improving air circulation, avoiding overhead watering, and using disease-resistant squash varieties.