The tomatillo, often called the Mexican husk tomato, is a popular garden vegetable known for its papery husk enveloping the fruit. When the vibrant green foliage begins to turn yellow (chlorosis), it signals an underlying problem disrupting the plant’s ability to produce chlorophyll. This yellowing is a symptom, indicating issues from nutrient imbalance to environmental stressors or biological attacks. Diagnosing the specific cause based on the pattern and location of the yellowing is the first step toward restoring the plant’s health.
Nutritional Causes of Yellowing
The chemical composition of the soil directly influences the plant’s leaf color, as certain elements are required for chlorophyll production. Yellowing that starts on the lowest, oldest leaves and progresses upward is a classic sign of nitrogen deficiency. Nitrogen is a mobile nutrient, meaning the plant pulls it from older leaves to support new growth, leaving the lower foliage a uniform pale yellow.
A different pattern, called interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins themselves remain green, often points to a magnesium or iron deficiency. Magnesium is another mobile nutrient, so its deficiency appears first on the older, lower leaves. The yellowing between the veins is distinct, and the leaf edges may sometimes curl upward.
If the interveinal yellowing appears exclusively on the newest growth at the top of the plant, the issue is likely an iron deficiency. Iron is an immobile nutrient, so the plant cannot relocate it from old leaves to new ones when supplies are low. Soil pH that is too high, or alkaline, can bind iron molecules, making them unavailable to the plant.
Environmental and Watering Stress
Water management is one of the most frequent non-nutritional causes of yellowing in tomatillo plants. Consistent overwatering saturates the soil, displacing the oxygen that the roots need to respire and absorb nutrients. This oxygen deprivation leads to root damage, causing the entire plant to wilt and the leaves to turn a soft, limp yellow.
Conversely, underwatering causes the plant to enter survival mode, resulting in leaves that turn dry, brittle, and crispy, often starting at the edges. While both overwatering and underwatering cause wilting, the difference in leaf texture and soil moisture is the diagnostic clue. Checking the soil several inches down before watering helps confirm the moisture level.
Temperature extremes and improper light exposure also contribute to leaf discoloration. Tomatillos are warmth-loving plants, and prolonged exposure to cold temperatures (below 50 degrees Fahrenheit) can cause leaves to turn yellow or purplish. Too much intense, direct sunlight, especially during a heatwave, can lead to sunscald, where the leaves develop bleached or scorched patches that later turn yellow.
Identifying Pests and Pathogens
Biological threats, including pests and diseases, can cause yellowing as a secondary symptom of the plant’s stress or tissue damage. Sucking insects like aphids and whiteflies feed on the plant’s sap, draining the fluid from the leaves and causing them to turn yellow or stippled. These pests are typically found clustered on the undersides of leaves and along the stems, often leaving behind a sticky residue called honeydew.
Fungal and bacterial diseases often manifest as yellow spots or lesions that eventually lead to widespread chlorosis. Early blight, a common fungal issue, causes target-like spots with yellow halos on lower leaves before the spots turn brown. Septoria leaf spot similarly causes small, circular yellow spots that eventually develop gray centers.
Viral diseases, such as the Physalis Mosaic Virus, present a systemic problem, causing a mottled pattern of yellow and green that affects the entire plant. This discoloration is often accompanied by stunted growth or distorted leaf shapes. Viral infections are incurable, and the infected plant must be removed and destroyed to prevent the virus from spreading to healthy plants.
Actionable Steps for Recovery and Prevention
Recovery begins by confirming the soil’s condition through a professional soil test, which identifies specific nutrient deficiencies and pH imbalances. If nitrogen or magnesium deficiency is confirmed, a balanced, water-soluble fertilizer or an application of Epsom salts can quickly address the issue. Adjusting the soil pH with amendments like sulfur or lime is necessary to ensure iron and other micronutrients become available for uptake.
To manage watering issues, establish a deep but infrequent watering schedule, allowing the top inch or two of soil to dry out between applications. Improving drainage is necessary for plants in containers or heavy clay soil to prevent root rot associated with overwatering. Applying a layer of organic mulch helps regulate soil temperature and conserves moisture.
For pest management, inspect the plant regularly, especially the undersides of the leaves, and use an integrated pest management approach. Small infestations of sap-suckers can be treated with insecticidal soap or neem oil spray, which disrupts the pests’ life cycle. If a fungal or bacterial issue is suspected, remove and discard affected leaves immediately and consider a copper-based fungicide to halt the spread.