Posts Tagged ‘horticultural oil’

Sooty mold on California laurel; Joseph O'Brien, USDA Forest Service, Bugwood.org

Last week we had a great question posted on our Discussion Board. It was interesting because it really involved two different issues, not just one.
Here is Kathy’s question:

I have two gardenias that are approximately 3 feet high that have a thick black mold/soot on all of the leaves. It is hard to wash off. On the underside of the leaves it looks like whitefly or the remains of whitefly. Should I cut them down to the ground now? I also have a yew in another section of the yard with the same black leaves. Thanks for your help.

Sooty mold on rose foliage

Sooty mold on rose foliage

What Kathy is seeing is the growth of sooty mold on the leaves of her gardenias. Sooty mold is a fungus but not one that causes disease so it is not directly harmful to her plant. However, heavy coatings of sooty mold may affect the plant indirectly by blocking sunlight and air from reaching the leaf surface and thus it can interfere with photosynthesis.

So why does sooty mold grow on the leaves and stems in the first place? This is the more important question because the reason it grows is the real problem for the plant.

Whiteflies congregate on the underside of a leaf

Whiteflies on the back of a leaf

The fact is that the growth of sooty mold usually indicates that some type of piercing, sucking insect has attacked the plant and these guys can do real damage if they are not controlled. It could mean her gardenias are infested with whiteflies (as she suspects), or even mealybugs, scales, or aphids to name a few others. When these insects suck the juices from the plant, they excrete a sticky substance called “honeydew” that drips down and coats the leaves and stems of the plant.

Fungal spores carried by the wind or rain become trapped on the sticky honeydew and germinate there. The fungus grows on the leaf surface using the honeydew as food and eventually covers the leaves with a black, sooty coating. Sooty mold may grow on any plant that is attacked by insects that excrete the honeydew that these fungi thrive upon.

Mark examines the underside of leaves for insect pests

Mark examines the underside of
leaves for insect pests.

What this means is that in order to solve a sooty mold problem, you first need to get rid of the insects that create the conditions which allow the mold to grow in the first place. The truth is that these are the critters that are going to cause the most damage to your plants.

If you notice sooty mold on your plants, you should carefully inspect them for sucking insects. Be sure to check the stems and especially the undersides of the leaves. Scales, aphids, mealybugs, spider mites, and whitefly are some of the most common sap sucking insects. Their feeding can damage the leaves and stems of a wide range of plants often causing stippling and curling of the leaves and distortion of the stems. In addition, some of these insect pests are vectors for certain viral diseases, spreading them from plant to plant as they feed.

Aphids have distorted these viburnum leaves.

Aphids have distorted these
viburnum leaves.

Bad news! Gotta get rid of ’em!

Fortunately, except for scale insects and severe infestations of whitefly, most of these “suckers” are not too hard to get rid of. Aphids, spider mites, and mealybugs can often be blasted off with a strong jet of water from the hose, although you may have to do this many times throughout the season to keep them at bay.

Many natural predators such as lady beetles and lacewings feed on all stages of these pests including scale insects and whiteflies. Be kind to your beneficials!

If the infestation is not too widespread, some selective pruning may take care of a lot of the problem. The pruned branches or leaves should be carefully bagged and thrown out in the trash; they should not be composted because many home compost piles do not get hot enough to kill all the stages of these insects.

Mealybugs like to hang out on the underside of leaves.

Mealybugs like to hang out on the
underside of leaves.

For an organic control, both Pure Spray Green and Bonide All Seasons Oil are listed for control of spider mites, mealybugs, aphids, scale, and whitefly. These products work by smothering the different stages of the insects. When you spray, it is very important that you coat the stems and especially the undersides of the leaves because these areas are where most of the insects are found. Be sure to read the label and apply accordingly. Sometimes a combination of selective pruning and spraying with horticultural oil works well.

Scale insects cover the underside of this Euonymus leaf.

Scale insects cover the underside
of this Euonymus leaf.

Of course as always, a healthy plant is less likely to be targeted by pests. Insects and diseases often attack plants that are weak or stressed due to drought or poor nutrition. Keep your plants healthy by fertilizing them and watering them properly.

Unfortunately, sooty mold will remain on the leaves for a while even after the insects are gone but eventually it will wear off. If the mold is particularly heavy on the leaves, you might try washing it off with a blast of water or wiping the worst off with a soft cloth.

Perhaps Kathy should be thankful for the sooty mold on her gardenia. It probably alerted her to a much more serious problem and may have saved her shrub from some serious damage!

Until next time – Happy Gardening!


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Peach totally engulfed by brown rot

Believe it or not, I followed my own advice this weekend!

All Seasons Oil will smother the overwintering egg cases of tent caterpillars.

All Seasons Oil will smother the egg cases of tent caterpillars.

I sprayed my fruit trees with Bonide All Seasons Oil. I have mentioned in many of our February and March newsletters and advised countless garden center customers that a late winter/early spring application of horticultural oil is one of the most important sprays to protect your fruit trees. It’s important because it smothers insect eggs (like tent caterpillar eggs) and overwintering insect pests (like codling moth larvae, mealybugs, and scale) by forming a coating of oil over them. It can also smother fungal spores and reduce the incidence of certain fungal diseases like rust or powdery mildew.

But have I ever sprayed it in our little orchard? No – at least not until this year! If it’s so important, why didn’t I do it? Well, my excuse was that whenever I had “time” to spray, the wind was blowing too hard or it was raining or I didn’t have the spray on hand or I missed the timing or …

Mealybugs cause damage to foliage and fruit.

Mealybugs cause damage to foliage and fruit.

In all honesty, it was more because it was so much trouble to mix up enough spray to cover all the fruit trees. Our little hand sprayer had to be refilled several times by the time we were finished. A few years ago, we got a backpack sprayer that held more and I used it for a while but even that had to be refilled quite a few times to cover all the trees and I never did manage to get that initial spray of horticultural oil on the them.

This year for an early birthday present, we bought ourselves a 16 gallon sprayer that can sit in the trailer we pull behind our mower. Best of all, it has a 12 volt pump that is powered right from the mower battery! Wow! I can tell that this new sprayer is going to revolutionize our fruit growing just like Uncle Bill’s old Troy-Bilt tiller revolutionized my vegetable gardening.

Brown rot ruins my peach crop.

Brown rot ruins most of my peaches before they have a chance to ripen.


The sprayer is made to attach to an ATV but Eric built a frame out of treated lumber that holds the tank and keeps it from moving around in the cart. Perfect!

We tried it out with the All Seasons Oil this weekend and it worked like a charm! We made up 9 gallons of spray which was the perfect amount to cover all the trees. So quick and easy!

I’m determined to keep up with the spraying this year so maybe we’ll get some fruit that we won’t have to share with the bugs and diseases!

Brown rot on a plum

Brown rot on a plum


At the top of my list is battling brown rot on our peaches. Every year, because I haven’t sprayed a fungicide, we watch as our peaches (and plums) become swallowed up by this dreaded disease. Brown rot is a common fungal disease that affects stone fruits like peaches, plums, apricots, and cherries. It can be devastating to a fruit crop and can destroy most or all of the fruit on a tree in a relatively short period of time.

Mummified peaches hang from one of my peach trees.

Mummified peaches hang from one of my peach trees.

Sanitation around the trees is one of the most important ways to try to reduce the incidence of this disease (and many other fungal diseases that affect fruit trees) because the fungal spores overwinter in plant debris and on mummified fruits that hang on the tree and fall on the ground. Even with careful cleaning and raking, treatment with fungicides is often necessary to help control brown rot especially if the disease has infected your trees in the past. To be most effective, it is very important to begin spraying for brown rot before infection occurs.

More information about brown rot

Black rot beginning to infect grapes.

Black rot beginning to infect a
bunch of grapes.

So … this weekend we’re going to fire up the sprayer again (weather permitting) to spray Bonide Liquid Copper fungicide on the trees and our grapes.

Bonide copper fungicide is a broad range fungicide approved for organic gardening that will help control a host of diseases in our orchard including brown rot on the peaches and plums, black rot on our grapes, and cedar-apple rust which affects our apples every year even though we have planted resistant varieties.

Cedar-apple rust infects some young apples

Cedar-apple rust infects some of
our young apples


I really think we’ll be able to keep up with the insects and disease this year! We’re even thinking of getting a two more peach trees and maybe another plum!

Now I’m just hoping that we don’t get a late frost that wipes out the blossoms!

Here’s to a “fruitful” year!

Until Next time – Happy Gardening
and Happy Spring!

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