What Causes Yellow Leaves on Knockout Roses?

Knockout Roses are landscape roses that take a lot of stress and worry from rose gardening. Even so, these practical, hardy roses may occasionally present with yellowing leaves. 

The leading cause of this problem is black spot disease. Even though Knockout Roses are very resistant to this disorder, they are not immune. This fungal disease can cause Knockout Roses’ leaves to turn brown or yellow during wet weather. 

yellow leaves on Knockout rosePin

Although black spot disease is one of the main causes of yellowing leaves in Knockout Roses, other problems can cause this unsightly symptom. 

Yellowed leaves are a very common warning that things are just not right with many plants. On the other hand, yellowed leaves can be part of the natural life cycle of the leaf. 

How To Diagnose Yellow Leaves In Knockout Roses

Examine your roses frequently. It’s important to provide consistent care and closely monitor your Knockout Roses’ health and condition. Close supervision will provide you with information to empower you when addressing yellowing roses. 

Check For These Common Problems Which May Cause Yellowing Leaves:

1. Too much water. During rainy, foggy, or humid weather, your plants may suffer from a lack of oxygen and/or the development of fungal infection. This can cause yellowing leaves.

2. Nutrient deficiencies. A lack of nitrogen is the classic cause of yellowing leaves. Plants need nitrogen for photosynthesis, and without it, green leaves turn yellow. Even so, you must take care not to overdo it with the nitrogen because too much nitrogen will cause lots of rose leaves and fewer flowers.

3. Unbalanced pH levels. Healthy roses like a fairly neutral soil pH level. A reading of 6-6.5 is perfect. If the pH level is below 6, your soil will tend to contain excessive amounts of phosphorus, nitrogen, and potassium. 

If it is greater than 7.5, it may lack phosphorus, manganese, and iron. Any imbalance can cause stress resulting in yellowing leaves.

4. Pest problems. Infestation by pests causes stress resulting in yellowing leaves and many more negative signs and symptoms.

A spider mite infestation can quickly cause yellowing leaves because these tiny pests suck the liquid out of your plant’s leaves. For this reason, it’s important to check the undersides of your Knockout Roses’ leaves several times a week. 

You are far more likely to detect pest problems early when you do this. This makes it easier to deal with them.

Other small pests to watch out for include aphids. The best control for aphids is to encourage insect predators and prune away affected parts of the plant whenever aphids are detected. You can also knock aphids off your plants with a powerful blast of water.

Rose slugs are Sawfly larvae. They are a common problem for rose keepers. You can deal with them by applying horticultural oil or insecticidal soap.

Japanese beetles can cause yellowing leaves. Luckily these pests are large enough to be easily seen and hand-picked away. When inspecting your plants, if you notice Japanese beetles. 

When you inspect your plants, carry a bucket of soapy water with you and pick off Japanese beetles and other large pests. Drop them in the bucket to be done with them.

5. Fungal disease is another common cause of yellowing leaves in Knockout Roses. Before the leaves yellow, you may notice some small black spots, which surround a yellowing circle. This is a black spot fungus. 

Powdery mildew is another fungal disease that can also cause yellowing leaves. However, before the yellowing begins, you will notice a light, powdery substance growing over the foliage. 

This causes the leaves to wrinkle, interfering with photosynthesis, thus causing yellowing leaves.

To avoid problems with a fungal infection, be sure to keep your rose bushes well pruned so that they get good air circulation. Avoid overwatering, and do not water overhead. Instead, soak the ground. 

When you cannot control the amount of moisture around your roses during very wet weather, you may wish to use a fungicide to help prevent fungal growth.

While some rose gardeners like to apply insecticides, miticides, and fungicides regularly in an attempt to prevent pests and fungal infections, this is not always a good idea. 

It is far better to inspect your plants frequently for signs of pests and fungal problems and deal with the problem only as needed than to bombard your plants, the air, and the surrounding soil with chemicals. Too much use of chemicals can have very negative and unintended effects.

6. Rose rosette disease (RRD) is a viral infection carried from plant to plant by eriophyid mites (Phyllocoptes fructiphilus) blown about by the wind. It is challenging to prevent this viral infection because the arrival of its carriers is so unpredictable. 

The virus is deadly and can kill a rose bush within a couple of years of infection. During this time, your sickly rose bush will naturally display yellowing leaves along with distorted growth and a host of other symptoms. 

There is no treatment for this disease. If you determine that your Knockout Roses have contracted RRD, you should destroy them. 

7. Stress: Yellowing leaves can signify all manner of stress in plants. Unpredictable changes in the weather can be a major cause of stress in plants. 

If the weather in your area is erratic, you may be able to help protect your plants against stress by providing consistent care. In extreme weather, your plants may need physical protection against excessive heat, sun, wind, and other environmental challenges.

8. Sometimes, leaves turn yellow and fall naturally. If you notice that the foliage on the lower portion of your Knockout Rose bush is turning yellow and falling, it may simply be that those leaves are not receiving as much sun as the leaves at the top of the bush. 

To prevent this from becoming an excessive problem, be sure to keep your rose bushes well pruned so that light can reach all levels of foliage fairly evenly.

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