5 Causes of Zucchini Leaves Turning Yellow and Rotting

When conditions are right, zucchini plants can be easy to maintain and highly productive. But if there is a problem, they might not ever make it to their fruiting stage. Signs of a problem include yellowing and rotting of leaves, stunted growth, and wilting of the entire plant. 

There are many techniques that can prevent these issues and treatments that can help your plants survive if they become afflicted. This article dives into those topics, along with the causes of zucchini leaves turning yellow and what they look like when plagued by these issues.

Why Does My Zucchini Plant Have Yellow Leaves?

There are several environmental factors that can cause the yellowing of leaves, but it’s always the first indication that there is a problem with your zucchini plants. Chlorosis is the term used when these environmental factors cause the leaves to produce an insufficient amount of chlorophyll, which is the green plant structure responsible for photosynthesis. When this happens, the green color provided by the chlorophyll disappears and the leaves appear pale or yellow. 

The most common environmental factors that can cause a zucchini plant’s leaves to turn yellow include inadequate watering, damaged roots, lack of sunlight, nutrient deficiencies, pests, and diseases. 

1. Watering Problems

Implementing an adequate watering schedule can be difficult for zucchini gardens because they need at least an inch of water per week but are sensitive to regular watering techniques. Overhead watering practices like sprinklers can lead to further issues including the spread of disease, and leaf damage from powdery mildew and sun scalding. 

Zucchini plants will often have a hard time recovering from constant under or over-watering, so keeping a consistent watering schedule is key to producing zucchinis throughout the entire growing season. 

Overwatering 

Overwatering can quickly cause the soil to become saturated, depleting it of precious oxygen and leaching it of nutrients. Many garden vegetables, including zucchini, are very susceptible to those flooded conditions and will quickly wilt and turn yellow. If the saturated conditions continue, then the plant will die. 

Overwatered zucchini plants will display slow growth, yellowing leaves, an increased chance of fungal development, and an overall wilted appearance. During the fruiting season, overwatering can lead to the leaching of calcium in the soil, which can cause Blossom End Rot. This common physiological disorder causes the zucchini fruit to turn brown and rot. 

Under Watering

Zucchini plants are sensitive to being underwater. Their vulnerable roots will quickly dry up, and if it continually happens, then the plant’s yield will be severely lacking. Underwatered zucchini plants will begin by wilting and then progress to severe leaf chlorosis if the conditions continue. When treating an underwatering issue, make sure not to overwater, which can quickly drain the soil of the nutrients needed for the zucchini to recover. 

2. Damaged Roots

Zucchini plants can experience yellowing leaves if their roots become damaged. This can be caused by severe compaction of the soil, digging from people or animals, or serious drought, all of which can diminish the zucchini plants’ large and sensitive root system. Prevent root damage by restricting foot traffic to garden paths, keeping the garden fenced, and adhering to watering schedules. 

3. Lack of Sunlight

A lack of sunlight can cause the yellowing of leaves on zucchini plants. They need between 6 to 8 hours of direct sunlight per day, or they will show signs of stunted growth, foliage discoloration, and a lack of fruit production. Crop placement during transplanting is very important to ensure the fruiting success of zucchini plants. 

4. Nutrient Deficiencies in Zucchini

Nutrient deficiencies are caused by the lack of nutrients like nitrogen, magnesium, and iron in the soil that are needed for plants to complete their metabolic activities related to growth and reproduction. If these nutrients are in short supply or are unavailable due to issues like pH imbalances, then it can cause issues such as stunted growth, poor fruit production, and chlorosis of foliage.

Iron Deficiencies

Iron deficiencies will show symptoms of the entire leaf yellowing with green veins remaining in the middle. Iron is often present in the soil but unavailable to the plant due to environmental conditions like over-oxygenation of the soil, pH imbalances, or chemical binding to other substances. As an essential nutrient for most organisms on earth, iron deficiencies will cause the zucchini plant to stop growing, cease fruiting, and quickly progress to leaf chlorosis and plant death.  

Magnesium Deficiencies

Magnesium is an essential nutrient that is involved with the enzymes, cell membranes, and tissue structures of the plant’s leaves, which is why deficiencies will show signs of foliage discoloration, stunted growth, and poor leaf production. Magnesium deficiencies will begin as yellow interveinal stripes in the youngest leaves, and if the deficiency continues, then every stem and leaf will develop yellowish-brown spots and reddish-purple tips. 

Nitrogen Deficiencies 

Deficiencies in nitrogen are common in zucchini plants. Symptoms include yellowing of leaves, beginning in older leaves and progressing to younger leaves as the deficiency worsens. This is often caused by a lack of organic matter in the soil, inadequate fertilizing, and nutrient leaching. Nitrogen is in greatest need during the flowering and fruiting phases, so be sure to add a nitrogen-rich fertilizer at the end of the vegetative phase. 

Mulching is beneficial for soil moisture retention and disease prevention, but as it decomposes it can steal nitrogen from the zucchini plants. Add a nitrogen-rich fertilizer to the soil if you choose to use mulching techniques.  

Nutrient Toxicity 

Plants may require macronutrients, but if they concentrate on high levels of the soil, then nutrient toxicity can occur. Chlorosis is the most common sign of nutrient toxicity. Over-fertilization and misuse of products are the most common causes of nutrient toxicity. The plant can usually recover from these imbalances if the concentrations were not high enough to burn the entire plant. 

5. Zucchini Plant Pests and Diseases

Zucchini plants are host to many pests and diseases, including spider mites, aphids, viruses, and fungi. Integrated Pest Management (IPM) is a scientifically proven way to control these unwanted pests and diseases without using excessive insecticides. Some of the techniques implemented in IPM include removing invasive plant species that attract pests, planting native species that repel or host pests, and creating shelter to attract predatory insects and birds. 

Spider Mites

Spider mites are tiny insects that can do big damage if left unmanaged. Spider mites will leave small yellow dots all over the surface of the leaves. As the infestation worsens the entire plant will begin to wilt and webbing will be evidently present on the undersides of the leaves. Spider mites are naturally plentiful in the environment, but an infestation can be easily prevented by spraying the leaves with mite-specific insecticidal soap, or an all-natural mixture that includes essential oils such as lemongrass and neem. 

Aphids and other Insects

Aphids, white flies, beetles, and borers can all be attracted to zucchini plants. All cases of insect damage will include evidence of leaf damage along with the presence of yellowing and browning leaves. Some of these pests deposit bacteria or fungal spores on the leaves during their visit to the plant, which can cause major infestations to occur. 

Cucumber Mosaic Virus

This virus will cause zucchini plants to become severely stunted and covered in blotchy yellowing spots on their leaves and fruits. This virus is transferred to the plant by aphids, so using IPM techniques like companion planting would be useful in preventing this disease. 

Fusarium Wilt 

This wilt is caused by a fungus, Fusarium sp., that is known to trigger Zucchini Plant Collapse by causing the zucchini plant’s leaves to wilt, turn yellow, and die. It is caused by the infestation of the cucumber beetle, which can be prevented using IPM techniques including all-natural essential oil sprays. If left untreated, this pathogen will rot the entire plant from the crown to the roots. 

Verdict: Zucchini Leaves Turning Yellow

There are several environmental factors that can cause the yellowing of leaves including inadequate watering, root damage, lack of sunlight, nutrient deficiencies, pests, and diseases. Overwatering can quickly cause the soil to become saturated, depleting it of precious oxygen and leaching it of nutrients. Zucchini plants are sensitive to inadequate watering and root damage, both of which will cause a lacked yield if it continues to happen. Nutrient deficiencies are caused by the lack of macronutrients like nitrogen, magnesium, and iron in the soil, which are often caused by an absence of organic matter, inadequate fertilizing, and nutrient leaching from heavy rain or overwatering. 

FAQ Yellow Leaves on Zucchini Plants