Zucchini plants can sometimes suffer from a common problem: their fruits turn yellow and rot before they are ready to harvest. This can be very frustrating for gardeners who want to enjoy their fresh zucchini.
What causes zucchini to turn yellow and rot? How can you prevent this problem and ensure a successful zucchini harvest? This article will answer these questions and provide solutions to help you grow healthy and delicious zucchini.
Understanding The Problem
Zucchini plants are a type of summer squash that can produce abundant and delicious fruits in the right conditions. However, sometimes you may encounter a problem: their fruits turn yellow and rot before they can be harvested.
This can be very disappointing and frustrating, especially after investing so much time and effort into growing these plants. To prevent this problem from happening or to deal with it effectively, it is important to understand the expected lifecycle of zucchini plants and the stages of fruit development. Understanding the appearance and causes of yellowing and rotting zucchinis is also important.
The typical lifecycle of zucchini plants consists of seven stages: seed sowing, germination, vegetative growth, flowering, pollination, fruit development, and harvesting.
Read more about the lifecycle of zucchini plants in our article “Exploring 7 Growth Stages Of Zucchini Plants“.
Why Is My Zucchini Turning Yellow And Rotting?
Zucchinis are susceptible to several issues that can result in yellowing and rot. The leading causes of this problem are improper watering, poor pollination, and fungal or viral diseases. Moreover, pest infestations, nutrient deficiencies, and insufficient sunlight can also contribute to zucchini turning yellow and rotting.
Pro Tip: Regardless of the zucchini turning yellow and rotting cause, it’s essential to promptly remove and dispose of the affected zucchini from the plant.
In the following sections, we will dive into each of these issues and explore the potential solutions to address them in detail.
What Makes Zucchini Yellow And Rotting
1. Improper Watering
Watering is one of the most essential aspects of zucchini care. Zucchini plants need a consistent and adequate water supply to grow healthy and produce quality fruits. However, watering too much or too little can negatively affect these plants.
Too much water can cause fruits to become soggy and rot, while too little water can cause fruits to become dry and wither. Both scenarios can result in yellowing of the fruits and eventually rot. Let’s discuss overwatering and underwatering in more detail.
Watering too much can create soggy and waterlogged soil conditions that can harm the zucchini plants. The excess water can reduce the oxygen levels in the soil, which can affect the root’s health and function.
The weakened roots can become vulnerable to diseases like root rot, which can spread to the rest of the plant. The symptoms of overwatering include yellow leaves, wilting, fruit yellowing and rotting.
To avoid overwatering, ensure that your soil has good drainage and does not retain excess water. You can use a well-draining soil mix or add organic matter like compost or manure to improve the soil structure. You can also mix perlite into the soil, which will drastically improve drainage.
Water your zucchinis regularly, but not too frequently. Let the soil dry out slightly between waterings. You can use mulch to help retain moisture in the soil. This will prevent waterlogging.
Pro Tip: If you notice zucchini rotting after a heavy rainy season, it could indicate insufficient soil drainage. While there may be limited immediate actions, consider creating multiple deep holes using a garden fork, long rod, or a garden knife to enhance drainage. For future growing seasons, focus on improving your soil’s drainage to prevent similar issues.
Watering too little can also cause problems for zucchini plants. When the soil is too dry, the plants cannot absorb enough water and nutrients. The plants become stressed and stunted, and the fruits may turn yellow, misshapen, and eventually rot.
To avoid underwatering, make sure your soil is moist but not wet. You can check the soil moisture by inserting your finger into the soil up to the second knuckle. If it feels dry, it’s time to water.
Water your zucchinis deeply and thoroughly, soaking the soil around the roots. Do this, especially during hot and dry weather, when the plants need more water. To retain the water in the soil and to avoid soil drying out, use mulch.
Water early in the morning when the sun is not strong enough to evaporate the water.
Pro Tip: If your soil is compacted, even with regular watering, your zucchinis might still suffer from water shortage. This issue is particularly true when growing zucchinis in garden beds. The force of gravity means that water will naturally seek the path of least resistance, which is usually at the side or a corner of the garden bed. To enhance water absorption in compacted soil, gently loosen the soil using a garden fork. Be cautious not to disrupt the zucchini roots excessively. By doing so, you’ll facilitate better water penetration, reducing the risk of water runoff and ensuring your zucchinis receive the hydration they need for optimal growth.
2. Poor Pollination
Poor pollination is another major reason why zucchinis may turn yellow and rot. Zucchinis have separate male and female flowers that need to be pollinated by insects.
Sometimes the pollination process may be hindered by various factors, such as a lack of pollinators, unfavorable weather conditions, or excessive use of pesticides. When the pollination is incomplete or insufficient, the zucchinis may not develop fully and eventually turn yellow and rot.
To prevent or remedy this problem, you can try to improve the pollination of your zucchini plants in two ways: attracting more insects to your garden by building a pollinator-friendly garden or manually transferring the pollen from one flower to another. Here are some tips on how to do both:
Many zucchini varieties depend on insects like bees, bumblebees, butterflies, or other insects for effective pollination. If your garden lacks these pollinators, zucchinis might not receive the necessary pollination. As a consequence, the fruits might not develop fully, exhibiting yellowing and potential rot.
One of the best ways to attract more insects to your garden is to plant flowers rich in pollen and nectar. Pollinators are drawn to colorful and fragrant blooms that provide them with food. Some of the best flowers for bees are lavender, borage, sunflower, clover, echinacea, and mint. You can plant these flowers near your zucchini plants or in pots and containers around your garden.
Another way to attract more insects to your garden is to avoid using pesticides that may harm them. Pesticides can kill or weaken the pollinators that visit your plants, reducing their numbers and activity.
Some pesticides may also contaminate the pollen and nectar that the insects collect, affecting their health and reproduction. If you need to control pests in your garden, you can use organic or natural methods that are safer for pollinators.
In cases where natural pollination is insufficient, you can resort to hand pollination. This manual process involves transferring pollen from the male to the female flowers, ensuring proper fertilization and fruit development.
By gently transferring a male flower’s pollen onto the stigma of a female flower using a small brush, you can significantly enhance pollination success and eliminate the risk of zucchini turning yellowing and rotting.
3. Fungal Or Viral Disease
Zucchinis can fall victim to various fungal and viral diseases such as blossom end rot, squash mosaic virus, fusarium wilt, and downy mildew. These can lead to yellowing and rotting of fruits.
Understanding these diseases and protecting your plants is essential for maintaining healthy zucchini. Let’s discuss each disease and how to deal with them in more detail.
Blossom End Rot
Blossom end rot is a common disorder caused by a calcium deficiency. It manifests as a dark, sunken area on the blossom end of the zucchini fruit, eventually leading to rot. Inconsistent watering, rapid growth, and fluctuations in soil moisture make this condition worse as calcium uptake is hindered.
To combat blossom end rot, ensure consistent and adequate watering. Mulching around the base of the plants helps regulate soil moisture. Consider adding calcium-rich amendments to the soil before planting to prevent this deficiency.
Pro Tip: Most of the time, there is plenty of calcium in the soil. It is other factors that stress the plant and reduce or impair the uptake and transportation of calcium to the fruit. Therefore, you don’t need to add amendments if your plant is, or was, recently stressed from drought, waterlogged soil, cold, or damage to the roots. 
Squash Mosaic Virus
Squash mosaic virus is a viral disease transmitted by aphids and by spotted and striped cucumber beetles. It affects a wide range of plants, including zucchinis. Infected plants often exhibit yellow mottling, stunted growth, and distorted leaves. The virus can lead to poor fruit development, resulting in underdeveloped and potentially rotting zucchinis.
Preventing aphid and spotted or striped cucumber beetle infestations through regular inspection and natural predator introduction can help mitigate the spread of the squash mosaic virus. Remove and dispose of infected plants to prevent further transmission. 
Pro Tip: While removing infected plants will reduce transmission, it might not eliminate them. Aphids and cucumber beetles will still have the disease in their system and can infect other plants. Therefore, the best control of squash mosaic virus is the control of pests that carry it.
Fusarium wilt is a fungal disease that affects zucchinis and other cucurbits by invading their roots and stems and blocking water transport within the plant. This leads to wilting, yellowing, browning of leaves, and overall plant decline. Fruits may also be negatively impacted, with the potential for premature rot or reduced quality and yield.
The disease is a soil-borne fungus that can survive in plant debris or on weeds for many years. The fungus can infect plants through root wounds caused by cultivation or nematodes or be introduced on contaminated seeds or transplants.
There is no effective cure for Fusarium wilt once a plant is infected. Prevention and management are the best strategies to reduce its impact. Some of the measures that can help prevent or limit Fusarium wilt are:
- Planting resistant or tolerant varieties of zucchinis.
- Practicing crop rotation and avoiding planting zucchinis or other cucurbits in the same area for at least three years.
- Keeping the soil well-drained and avoiding overwatering, as the fungus thrives in wet conditions.
- Remove and destroy infected plants as soon as symptoms are noticed to prevent further spread of the fungus.
- Never compost infected plants!
- Sanitize tools and hands with soapy water or alcohol after handling infected plants to avoid mechanical transmission.
- Keep your garden weed-free, as some weeds can harbor diseases.
- Use certified seeds for sowing to eliminate any possible seed-borne infections. 
Downy mildew is a foliage disease caused by a fungus. It is spread from plant to plant by airborne spores. It thrives in cool and humid conditions, especially when there are fluctuations between dry days and moist or humid nights.
It presents as pale green or yellow angular spots on the upper leaf surface, often accompanied by a gray or purple fuzzy growth on the underside. Severe cases can lead to leaf death and hinder the plant’s ability to produce healthy zucchinis, which could potentially rot.
If detected early, applying fungicides labeled for downy mildew control can help limit its impact. To prevent downy mildew, provide good spacing between plants for good air circulation. Water the plants at the base to avoid wetting the leaves, as moisture promotes the spread of the disease.
4. Pest Attacks
Pest attacks can wreak havoc on zucchini plants, leading to stress, damage, and potential fruit rot. Understanding these common pests and implementing appropriate control measures is crucial for preserving the health of your zucchinis.
Aphids are tiny pear-shaped, soft-bodied insects that can quickly multiply and infest zucchini plants. Depending on the species, they can range in color from green, black, red, yellow, brown, or gray. They have piercing-sucking mouthparts that are used to pierce plant tissue and remove sap.
Aphids can cause distorted growth, yellowing leaves, and a decline in overall plant health by injecting toxins into the plant, reducing photosynthesis, and transmitting viral diseases. This could potentially lead to zucchini rotting or reduced yield and quality.
You can spray plants with a strong stream of water to dislodge aphids from the leaves and stems. If the infestation persists, you can also use organic products such as insecticidal soap or neem oil. These products can kill aphids on contact without harming beneficial insects. 
Pro Tip: If you are using insecticidal soap or neem oil, spray the bottom and top of the leaves and repeat the application as per instructions.
Read more about aphids in our article “Aphids – Identification and Control“.
Spider mites are microscopic pests that belong to the arachnid family, related to spiders and ticks. They have eight legs and are about the size of a grain of sand or a period drawn with a pencil. Depending on the species, they can be yellow, green, red, or black.
The most common species that attacks zucchinis is the two-spotted spider mite, which has two dark spots on its body. When spider mites puncture plant cells to feed on their contents, they cause stippling, yellowing, bronzing, or browning of leaves. They also produce fine webbing on the undersides of leaves or on stems. Spider mites thrive in hot and dry conditions and can reproduce rapidly.
Severe infestations can lead to leaf drop, stunted growth, and reduced yield and quality of fruits. On rare occasions, spider mites can also cause fruit rot by creating entry points for secondary infections by bacteria or fungi.
Increase humidity around zucchini plants by misting them regularly. This will discourage spider mite activity and reproduction. Introduce predatory mites, such as western predatory mites or Phytoseiulus, to control spider mite populations naturally. These beneficial insects can feed on spider mites and reduce their numbers. , , 
In severe cases, you can use insecticidal soap or horticultural oil to kill spider mites on contact without harming beneficial insects.
Squash bugs are dark gray to brown flat, oval-shaped insects. They are one of the most common and troublesome pests of zucchini and other cucurbits, especially squash and pumpkins.
They use their piercing-sucking mouthparts to feed on the sap of leaves, stems, and fruits, causing wilting, yellowing, browning, and eventual plant death. They also inject a toxin into the plant that can cause rapid wilt.
Squash bugs can damage the fruits by creating scars and entry points for secondary infections by bacteria or fungi.
Handpick squash bugs and their eggs from the leaves and stems regularly. The eggs are oval-shaped, yellowish to bronze, and laid in small clusters on the undersides of the leaves.
Using row covers early in the growing season can help prevent infestations by excluding adult bugs from laying eggs on the plants. However, you need to remove the row covers when the plants start to flower to allow pollination. 
Neem oil or insecticidal soap can also be effective if applied to affected areas when the bugs are young and before they develop resistance.
Squash Vine Borers
Squash vine borers are caterpillar larvae of a winged moth that bore into the stems of zucchini plants and other cucurbits, especially those with hollow stems. They cause wilting, yellowing, and eventual plant collapse by blocking the water and nutrient transport within the plant. They also create entry points for secondary infections by bacteria and fungi that can cause rotting.
Protect the base of zucchini stems by wrapping them with lightweight fabric, aluminum foil, or cover the whole plant with a floating row cover to prevent egg-laying by the adult moths. Just remember to remove it when the plants start to flower. This will let the pollinators reach the flowers and pollinate them.
You can also apply insecticides labeled for squash vine borer control when the moths are active.
If borers are detected, carefully slit affected stems to remove the larvae and then mound soil around the base to encourage new root growth.
Pro Tip: Avoid using floating row covers if you have grown zucchinis or other squash-related plants in the same spot last year. Squash vine borer eggs might be left in the soil that can hatch and attack your plants. , 
5. Nutrient Deficiencies
Zucchini require a balanced supply of nutrients and appropriate soil conditions for optimal growth. Nutrient deficiencies and imbalanced soil can contribute to the zucchini turning yellow and rotting.
Some of the most common nutrient deficiencies in zucchini plants are nitrogen, phosphorus, potassium, iron, and calcium. These nutrients are essential for various plant functions, such as leaf growth, fruit development, photosynthesis, and disease resistance.
Symptoms of nutrient deficiencies include pale or yellow leaves, stunted growth, poor fruit set, fruit deformation, and reduced yield and quality.
Conduct a soil test to determine nutrient levels and pH. Based on the results, amend the soil with organic matter, compost, or appropriate fertilizers to ensure that zucchinis receive essential nutrients. For example, you can use a balanced fertilizer with an N-P-K ratio of 4-4-4 or 4-6-4.
Regularly monitor soil health and adjust nutrients as needed to prevent deficiencies and promote healthy plant development. You can also use mulch to conserve soil moisture and to avoid nutrient leaching.
6. Not Enough Sunlight
Sunlight is crucial for the growth and productivity of zucchini plants. Zucchinis are warm-season crops that need at least 6 to 8 hours of full sun exposure daily to thrive. If your zucchinis are not receiving enough sunlight, it can lead to weakened plants, reduced fruit development, and potentially zucchini turning yellow and rotting.
Sunlight is essential for photosynthesis, which is the process by which plants convert light energy into chemical energy and produce sugars that fuel plant growth. Without enough sunlight, zucchini plants cannot produce enough sugars. They may suffer from stress, stunted growth, poor fruit set, and lower yields.
While you cannot improve sunlight conditions in the middle of the growing season, you can prepare better for the following year.
To ensure a successful harvest next year, choose a sunny spot for planting your zucchini seeds or seedlings. This spot in the garden should receive a minimum of 6 to 8 hours of direct sunlight each day. This will promote healthy growth and minimize the risk of yellowing and rot.
You can also avoid planting zucchinis near taller plants or structures that may cast shade on them during the day.
Growing zucchinis can be a rewarding and enjoyable experience, but it also comes with some challenges. One of the most common problems that you can face is the zucchini turning yellow and rotting of their fruits.
This can be caused by various factors, such as poor pollination, excessive or insufficient watering, nutrient deficiencies, diseases, pests, and lack of sunlight. To prevent or solve these issues, you must understand and address the underlying causes and take proactive measures to ensure optimal growing conditions for your zucchinis.
By doing so, you will not only harvest vibrant and healthy zucchinis but also cultivate a deeper connection to your garden and nature. Each zucchini you pick will be a testament to your commitment, care, and a source of joy and satisfaction.
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