Petunia leaves turning yellow

Yellow Leaves On Petunia Plants: Why A Petunia Has Yellow Leaves

Petunias are beloved, no fuss annual plants that most gardeners can’t do without in the landscape. These plants are steady performers in summer, rewarding our neglect with bountiful flower displays and few pest and disease problems. Occasionally, however, a specific issue such as yellowing petunia leaves, can leave a gardener scratching his/her head.

Why a Petunia Has Yellow Leaves

In many cases, yellow leaves on petunia plants are cultural in nature but sometimes the cause is a common disease that can be spread from cucurbits. Some information on the exact symptoms and causes can help you determine if your petunia is destined for the rubbish bin or if you can save the plant to bloom another day.

Petunias need well drained soil, bright sunlight and warm temperatures for best performance. These annuals come in a variety of petal formations, providing an ideal appearance for any type of flower display. When you see leaves on petunias turning yellow, it is important to note the pattern of fading. Some patterns indicate a destructive virus that can infect other plants in the garden, as it is transmitted through the feeding activities of aphids.

How can you tell if a virus is causing yellowing petunia leaves or if you simply need to water or fertilize? The word “mosaic” is a clue.

Cultural Causes of Yellow Leaves on Petunia

Petunias need plenty of water but they don’t like their petals and leaves to remain wet. This can cause them to wilt and occasionally discolor. Water from the base of the plant deeply and allow the soil to dry out in the top few inches before drenching the plants anew.

Soil that doesn’t percolate leaves roots soggy and unhappy. Make sure your soil is a well-draining mix. Potting mixes should be half peat moss and half soil. The peat moss will provide adequate acidity to these plants. You should perform a soil test before installing in-ground plants to ensure adequate acidity. If the test comes back too alkaline, add some lime before planting petunias.

Nutrient Deficiencies Causing Petunias with Yellow Leaves

Young petunias need plenty of nitrogen to force green leaf and shoot development. When grown in nitrogen poor soil, older leaves will turn greenish yellow or even fully yellow. Chlorosis in the veins of leaves may indicate a potassium deficiency. When a petunia with yellow leaves has necrotic spots at the veins after fading, a higher dose of potassium is required.

Magnesium deficiency creates a similar condition on the newest leaves. Young leaves on sulfur deprived plants are decidedly greenish yellow. Micronutrient deficiencies that can cause yellowing petunia leaves are boron, manganese and iron. Iron is the most common deficiency in many regions. A soil test can help determine which nutrients need to be administered.

Leaves on Petunias Turning Yellow Due to Disease

The most probable cause of petunias with yellow leaves is tobacco mosaic virus. Here is where the indicating word “mosaic” can help identify the disease. A mosaic is a collage of patterns which create an image. In the case of the yellowed petunias, the mosaic shows as mottling in golden yellow. It almost seems purposeful but instead is an indication that your plant has been infected with TMV.

This virus affects cucumbers, tobacco and other plants. It is transmitted through aphids but also in soil and through the hands of tobacco users. Once your petunias have the virus, there is no cure and they should be thrown out. Do not add them to the compost pile, as average temperatures are not high enough to destroy the disease and you could inadvertently spread it around your garden.

Caring For A Petunia

Petunias are well known for being low-maintenance flowers. They also stay beautiful from spring to fall and come in a variety of styles and colors. Growing petunias is relatively simple and the pay-off is a rewarding spectacle. You can start seeds inside or out, though it tends to be easier inside since you can keep a better eye on them and cater to their needs. Professionals suggest buying fully-grown petunias from a gardening store and re-planting them wherever you desire. This will give immediate results, but you will find that there are just as many problems with re-planting as with seeding.

Choosing the right petunia is key in successful growing because some are susceptible to disease, making all of your work ruined. Grandifloras are petunias that you have to pay close attention to. The have the largest bloom, 3 to 4 inches in diameter, but in hot and humid summers you will notice that this specific variety will get damaged much easier and it runs the risk of rot (fungal infection). Actually, if you are in a hot and humid climate, you probably know that fungus will form on most any type of plant, so look for this. Petunias are also susceptible to iron deficiencies. If you notice the leaves turning yellow and new leaves are yellow, don’t ignore these signs.

Fertilizing and Watering

Before we delve straight into fertilizing, you most know how to read the fertilizer label. You will generally find three numbers. In the case of petunias, you want to use 8-8-8, 10-10-10 or 12-12-12 fertilizer. These numbers are telling you the amounts of nitrogen, phosphorus and potassium measured by percentage by weight. In other words, you want a balanced fertilizer for your petunias. Easy enough!

Work some of this balanced fertilizer into the soil (generally around 2 lbs. per 100 sq. feet). It is best to add fertilizer soon after planting, which will be early spring after the last frost. Around mid-July you should start using a liquid fertilizer. For boxed petunias, or those in small containers, use a time-release fertilizer as well as liquid, and water the plants often, sometimes every day, to replace lost moisture. For ground cover petunias, use liquid fertilizer every week and water a few times each week. It is no surprise that this variety is quite stingy with nutrients. For all other petunias, liquid fertilize every 2 weeks and set a sprinkler out to water them once or twice a week. The ground should be wet on the top 6 inches of the soil.

Death

If your petunias are planted in the ground and not in a movable container, they will die with the first frost. If they are in a container, you can take them inside where they can live for another month. They will not be able to withstand much more of the cold temperatures or survive with small amounts of sun. Remember to remove your plants after they’ve died because rotting petunias can adversely affect the soil for next year’s plants.

March 2014

Greening Up Calibrachoa & Petunia
By Erik Runkle and Paul Fisher

Many popular spring garden plants, such as calibrachoa and petunia, perform best at a low media pH, around 5.4 to 5.8. When the pH becomes too high, above 6.2 for these crops, iron in the root zone becomes less available to these plants. The most common symptom of a high media pH is light green or yellow color between leaf veins, which is referred to as interveinal chlorosis (Figure 1). If the media pH continues to rise, the symptoms become more severe and can include bleaching of the newest leaves and leaf necrosis (tissue death). Once necrosis occurs, the only remedy is to grow new, healthy leaves to cover the damage.

Maintaining a desirable media pH should be one of the most fundamental goals for a grower. Simple in-house media tests should be conducted weekly to determine the pH and electrical conductivity (EC) for various crops. Over time, a change in these values can alert a grower to take small corrective actions to reverse undesirable trends. For example, if the media pH increases above the desired range, the type of fertilizer could be changed to one that has a more acidic reaction (with more ammoniacal nitrogen), or acid could be injected (or increased in dosage) to the irrigation water to reduce its alkalinity. Testing EC is important because chlorosis may also result from inadequate fertilizer, which will lead to a low EC.

Unfortunately, routine in-house soil testing is still not that common among commercial growers. Therefore, media pH problems usually become evident when crops show symptoms, which in most cases were several weeks in the making. When the symptoms are moderate in severity, a corrective action needs to be taken in an attempt to make the crop saleable. Although a high media pH is the most common cause of plant chlorosis, a poor root system or a root disease such as Pythium can also be to blame. Therefore, examine the roots and test the media pH before proceeding with corrective actions.

If the media pH is high and a quick fix is needed, one effective option is to add iron as a substrate drench. Iron EDDHA is the most effective form of iron because it is soluble regardless of pH. Some of the products that contain iron EDDHA are Sequestrene 138 and Sprint 138. A suggested drench application rate of iron EDDHA is 5 ounces per 100 gallons of water, which provides 22 ppm of iron. This solution should be applied with generous leaching, followed immediately by rinsing off the foliage to avoid leaf burn. A single application typically “greens up” plants within a week. If needed, a second application can be made.

Not all forms of chelated iron are equally effective when the media pH is high (Figure 2). EDDHA is soluble at a wide pH range, iron DTPA is less soluble at a high pH, and iron EDTA is even less soluble. Therefore, iron EDDHA is the preferred form, and generally iron EDTA is marginally effective unless pH is below 6.2.

In addition to calibrachoa and petunia, crops that perform best at a low media pH include bacopa, diascia, lantana, nemesia, pansy, scaevola, snapdragon and vinca. If these plants become chlorotic, you can follow the same pH reduction strategies and/or apply an iron-EDDHA drench to produce higher-quality crops. Be careful not to apply iron to iron-efficient crops that grow best at a high pH (above 6.0), such as marigold and geranium, because that can lead to irreversible iron toxicity.

Erik Runkle and Paul Fisher

Erik Runkle is associate professor and floriculture extension specialist in Michigan State University’s department of horticulture and can be reached at Paul Fisher is professor and floriculture extension specialist in University of Florida’s department of environmental horticulture and can be reached at

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