Wet weather can spread common diseases to roses

Q: I have several Lady Banks roses planted in the raised bed along the wall. I noticed that several of the leaves have little dots along the edges with a clear, sticky substance coming from them. I can’t see any bugs, but my fingers are sticky when I touch the leaves and stems. Today I noticed that the leaves are drying and curling. What do you think is causing this and what can I do to control it?

A: Diseases are ready to spread any time the weather has been wet. Sometimes these diseases happen soon after it rains and sometimes months later. To me, this sounds like black spot or Cercospora leaf spot of roses. These both are common to roses in wetter climates and after rain. It is important to get rid of any leaves that drop after the infection starts.

For a home remedy, try using 1 to 1½ tablespoons of baking soda mixed in a gallon of water, with 1 tablespoon liquid soap added to help penetrate the leaf surface. If that fails, use either Neem oil for disease control or a conventional pesticide for black spot or Cercospora of roses that you can buy at your local box store or nursery.

An alternative is to prune diseased and damaged stems back to eliminate the diseased leaves, being sure to disinfect the pruners between each cut. Rake mulch and debris away from the bush and place them in the garbage.

Pruning this way causes the plant to regrow. Protect new growth during wet weather with any type of fungicide for roses for best control. Do this any time we have wet weather again.

Q: I am concerned with the condition of my pear tree.

A: Based on the photo you sent, this is definitely fireblight of pear. Most European pears, nearly all Asian pears and some apple varieties are susceptible to this bacterial disease.

From the look of your picture, it looks like this particular disease was present in this tree back in 2022. The way I can tell is how far advanced it is in only May of this year. New infections are not this advanced at this time of year.

May is when we start seeing fireblight, not the advanced blackened version we see in your version. It looks like a textbook example of this disease when it is advanced.

Prune out the infection by taking out 8 to 12 inches of the uninfected wood. If this particular disease is in the trunk of the tree, then the tree is a goner and should be removed. This disease is particularly virulent or spreads very easily. Get any prunings, or the tree itself if you find it is infected, off the property and disinfect the pruning blades after each cut. I guarantee it will spread.

Remember the difference between signs and symptoms? The blackened fireblight of this tree is a symptom of this disease while the infection itself is a sign. Symptoms are what I am basing my decision on and the fact these symptoms are found on a pear tree.

Q: I have one of three rose trees that died. I noticed that a new tree seems to be growing at the bottom of a dead one. Should I let it go and see what happens?

A: You can if you want. It won’t harm anything. If suckers grew from the base of a rose tree, then the tree rose probably died during the winter. I don’t know what happened to kill it, but the top part is probably dead.

My experience with tree roses is that they die usually from watering too frequently and poor soil drainage of water from around the roots. Roses of all types should not be watered daily. They should get water every other day at best.

Roses do particularly poorly when surrounded by rock. They grow much better when they are planted in organic soil, and this soil drains water easily. In our climate that usually means the soil around them is covered in wood chips rather than rock.

Q: I found this strange growth near our house in the desert. A very thin chalky white exterior with fine dirt inside. Any idea what this might be?

A: It is probably one of the slime molds. It is in the group called Myxomycetes. They are common in the desert a few weeks after a rain. They are also common in many home landscapes built on top of desert soil.

They are amoeba-like and can take many different forms. I have been asked to identify slime molds that look like vomit to some that look like mushrooms.

Q: My Meyer lemon tree produced fruit during the last two years but no fruit this past year. How come?

A: I know I sound like a broken record, but it was probably because of freezing weather. Could be a disease, but that is a stretch. It just takes temperatures in the mid-20s for a few minutes and open flowers to freeze to death. Several of those temperatures in a row, a few weeks apart, and you have no fruit.

That’s why I encourage you to have a local thermometer or wake up just before sunrise and see if there is ice on the water. It will be gone with a little bit of sunlight.

It doesn’t have to be citrus, but any open flower will be killed. It happens even to early peaches (Earlitreat) and apricots (Earlicot) too. Some peaches flower as early as the first week of February and early apricots the second week of February.

If it freezes Feb. 15, we have less fruit. If it freezes a couple of weeks apart, then we may have zero fruit. That’s why we don’t plant things that have important flowers until March in our climate if we want the fruit.

Once the fruit has set, the temperature threshold becomes colder than mid- to high-20s or more for loss of fruit. It’s just the open flowers that are the most affected.

Q: I read your article about stopping olives from growing and had two questions. Is there a solution I can apply to the base of the tree to stop the olives? If not, can you recommend a sprayer or solution that will reach the top of the tree if I use Florel? I am having a tough time trying to figure out what to do.

A: No, there is nothing you can apply to the European olive at the base of the tree and have it stop flowering. Currently, there are two products available to prevent fruit formation: Florel, which you mentioned, and Olive Stop, which contains NAA (naphthalene acetic acid).

The flowers must be open at the time of spraying for these two products to work because the spray must come in contact with the ovary of the flower to be effective. It kills the ovary. That’s why the label says to spray it twice — once at 20 percent of full bloom and the second time at 80 percent — for total control.

Most spray companies spray it once at about 50 percent of full bloom. I am not being critical of these companies, but that does leave opportunities for homeowners such as yourself. Spraying at 50 percent of flowering leaves a small amount of fruit untouched by the spray, and the fruit survives as green and eventually black olives.

We used to use Maintain CF125 as a bark spray combined with a crop oil to get it inside the tree. Maintain CF125 could be sprayed on the trunk about a month before the tree flowers, and it would drop the flowers.

But Maintain CF125 is labeled primarily for turfgrass and some ornamental trees. Sorry, there is nothing you can apply to olive trees to stop them from flowering — fruiting yes, but not flowering.

Q: I have a Western redbud planted in a very hot location near a block wall. The heat took a toll on it last summer, but it survived and is currently growing vigorously.

I am concerned, however, about the coming high temperature and the radiant heat the wall produces in full sun. Would covering the wall with shade cloth while leaving the tree exposed be of any help? Would the color of the shade cloth be important?

A: Western redbud is a desert tree, but it is also a tree found in desert valleys. That means it likes shade in the late afternoons. It can go without water for a long time, but it can’t handle the heat like a foothills palo verde.

The darker colors radiate more heat than light colors. Covering the wall in burlap or reed fencing helps. You can get reed fencing from box stores like Lowe’s or Home Depot.

Western redbud is found in desert valleys so it will need protection at first until it gets larger and creates its own shade.

Sounds like the Western redbud you bought is in direct sun and heat, and so it is burning up. This fall (October) when it cools off, move it farther from the wall when possible.

It is best if the wall behind the tree is covered with a vinelike cat’s claw. The green foliage will help keep the tree cooler.

Bob Morris is a horticulture expert and professor emeritus of the University of Nevada, Las Vegas. Visit his blog at xtremehorticulture.blogspot.com. Send questions to Extremehort@aol.com.