Tomatillos, common relatives of the tomato plant, are generally robust producers, but they often signal distress through a loss of green color in their leaves. This yellowing, known scientifically as chlorosis, is a common symptom indicating an underlying problem that needs diagnosis. Understanding the location and pattern of the yellowing, along with the plant’s overall environment, is the first step in determining the cause and developing a precise treatment plan.
Yellowing Due to Water and Nutrient Imbalances
Improper watering is the most frequent cause of chlorosis, affecting the plant’s ability to take up oxygen and nutrients from the soil. Overwatering is often the culprit, as saturated soil suffocates the roots, leading to root rot and preventing proper function. This root stress causes generalized pale yellowing across the plant, often accompanied by wilting or drooping leaves. Conversely, severe underwatering also stresses the plant, leading to yellowing as metabolic processes slow down, potentially causing leaves to become brittle or drop.
When water levels are balanced, nutrient deficiencies become the next likely cause, signaled by where the yellowing appears on the plant. Nitrogen and magnesium are mobile nutrients, meaning the plant can move them from older leaves to support new growth. Nitrogen deficiency causes a uniform pale yellowing that starts on the oldest, lowest leaves and progresses upward. Magnesium deficiency also affects older leaves first, but presents as interveinal chlorosis, where the tissue between the leaf veins turns yellow while the veins remain green.
Deficiencies in immobile nutrients, such as iron, show symptoms on the newest growth because the plant cannot relocate these elements from older tissues. Iron deficiency appears first on the youngest leaves at the plant’s top, often presenting as interveinal chlorosis. Note that a nutrient deficiency is not always due to a lack of the nutrient in the soil. Sometimes, an incorrect soil pH level prevents the roots from absorbing the nutrient, effectively locking it out from the plant.
Identifying Pests and Diseases
Yellowing not caused by water or nutrients may be due to living organisms, requiring a different approach for identification and treatment. Pests that feed on plant sap directly stress the tomatillo, leading to localized or generalized chlorosis. Tiny sap-sucking insects like spider mites cause a speckled yellowing pattern, called stippling, and may leave fine webbing on the undersides of leaves. Aphids also feed on sap, leaving behind a sticky residue called honeydew, which can cause leaves to yellow and curl.
Diseases, often caused by fungi or bacteria, typically present with additional symptoms that distinguish them from simple nutrient issues. Early Blight, a common fungal issue, begins as small yellow spots that enlarge and turn brown, often with a target-like appearance. Fungal and bacterial infections, such as those causing wilts, can inhibit the plant’s vascular system. This inhibition leads to yellowing, wilting, and sometimes discoloration or cankers on the stems. Removing infected leaves and ensuring good air circulation are the first lines of defense against these biological threats.
Environmental Factors and Plant Age
External physical stressors can also trigger a yellowing response not related to soil chemistry or biological attack. Sudden temperature fluctuations can induce temporary chlorosis, such as a cold snap causing leaves to pale, or extreme heat leading to sun scald. When a young plant is moved into the garden, it may experience transplant shock, causing temporary yellowing as the root system adjusts to the new conditions.
A different, less concerning type of yellowing is part of the plant’s natural life cycle. As the tomatillo grows and begins to focus energy on producing fruit, the oldest, lowest leaves naturally yellow and die off. This process, known as senescence, is harmless and indicates the plant is efficiently redirecting resources to new growth and developing fruit. If the new growth remains green and healthy, the loss of these oldest leaves is typically not a concern.
A Step-by-Step Guide to Recovery
To effectively treat a yellowing tomatillo, a systematic diagnostic process is necessary, starting with an examination of the plant’s structure. Determine the location of the chlorosis: yellowing on the oldest, lower leaves suggests a mobile nutrient deficiency or a watering issue. Yellowing on the newest, top leaves suggests a deficiency in an immobile nutrient like iron.
Next, check the soil moisture level by inserting a finger two inches deep; soggy soil indicates overwatering, while powder-dry soil suggests underwatering. If moisture seems correct, inspect the undersides of the leaves carefully for pests, looking for stippling, sticky residue, or webbing. If the plant shows spots, lesions, or stem discoloration, a disease is the likely cause, and affected material should be promptly removed to prevent spreading.
Based on the diagnosis, implement the appropriate treatment plan immediately. If watering is the issue, adjust the frequency to allow the top inch of soil to dry slightly between waterings. For a suspected nutrient deficiency, apply a fertilizer that targets the specific deficiency, such as a nitrogen-rich formula for lower-leaf yellowing. As a preventative measure, consider having a soil test done by a local extension office, which provides an accurate analysis of nutrient levels and soil pH for long-term soil health.