Rainy conditions can cause diseases in plants

It rained this past week. That’s good news, right? Maybe.

If you are growing Asian pears, European pears like Bartlett or apples, you might see a disease pop up beginning around May. The infection starts as black dieback of new growth like it was burned by fire. This disease is called fire blight and can lead to tree death if not controlled when it’s first seen in May or June. It can be common several weeks after spring rains, particularly if trees were flowering during spring rainy weather.

Fire blight is a bacterial disease, so fungicide sprays won’t work. The best thing is to remove the infected limb by pruning generously. Sanitize the blades of any pruning tools with 70 percent alcohol after each cut. Then bag this diseased black growth and get it off the property ASAP.

Another disease I saw pop up last year in grapes was bunch rot, but it was not really seen until the bunches got big. However, the disease started in the spring with rainy weather like we just had. For homeowners, copper-containing fungicide sprays work best to contain this disease with the first of three sprays starting now as the grape bunches start to form.

Q: I recently purchased a Lila dwarf avocado tree. I’ve never heard of an avocado tree in Southern Nevada, but with warming temperatures and a tree that is supposedly cold-resistant down to 15 degrees, I’m giving it a run. Are you familiar with cold-resistant avocados, and do you know of any living in Southern Nevada?

A: You have the right attitude about growing avocado here. Give it a go, but don’t expect miracles and enjoy it while you can. If you’re thinking about avocado, think about citrus as well. Both grow in the same climate zone, which is subtropical. If your neighborhood has grown citrus for several years, then you can probably grow this avocado at least for a while.

Scientists classify avocados into three types: Mexican, Guatemalan and West Indian. Some of our favorite avocados from the grocery store are Mexican types. Lila has good fruit but a bit smaller than you would buy from the store.

Lila (aka Opal) is one of the cold-hardy varieties of the Mexican type. So instead of getting damaged at freezing temperatures like many other avocados, Lila can handle temperatures down to near 15 degrees F for a short time when it’s fully mature. More reasonable low-temperature estimates might be the low 20s for it and others like it. Other Mexican varieties like Joey, Fantastic, Opal, Poncho and Mexicola Grande can be grown in areas where there are low winter temperatures as well.

The next good feature of Lila for backyards is its height. It is one of the smallest cold-hardy avocado trees you can buy. It is considered a semi-dwarf avocado tree that can grow to a height of 20 feet, but you could keep it pruned down to around 10 feet tall.

You need a second avocado tree for good fruit production. If you plant only Lila, you will get some fruit but not as many as if you get an avocado pollinator tree.

Q: I was reading your blog, Xtremehorticulture of the Desert, about spider mite damage on Italian cypress. You recommend a miticide spray. Which one should I get?

A: All cypress trees have problems in desert landscapes. Spider mites are common problems with Italian cypress during the heat of the summer. If you have Italian cypress, it’s best to wash the foliage off with a high-pressure hose once a month during the summer. This hosing of the green foliage gets rid of the dirt that covers the foliage and increases the chance of spider mite breakouts.

If Italian cypress starts turning gray first and then brown due to feeding from spider mites, you will not have much choice but to spray something. If it’s not too late, start with soapy water sprays weekly to remove dust or debris interfering with predatory mites. Predatory mites control spider mites, but the dust interferes with their predation. Or use plant oils such as neem or cinnamon oil in early-morning hours and retest the tree to see if it worked.

Spraying the tree with a miticide is probably a good decision if nothing else has worked. Miticides are insecticides that are good at controlling mites. Select an insecticide from the shelf at your nursery or garden center that says it controls mites on the label. The active ingredients on the label of the insecticide might include chemicals like azobenzene, dicofol, ovex or tetradifon, among others.

Unfortunately, spraying with a miticide might cause you to spray again because of other pest problems. That’s why I tell you to try other methods first.

Q: The fruit on my tangerine tree was very dry when I harvested it. My tree is about 10 feet tall when I purchased it in a 24-inch box. I had it for two years, and both years the fruit has been very dry. The tree looks healthy and blooms good, but the fruit is just not edible even when I tried to pick at different times.

A: When you plant anything from a 24-inch box make sure the soil surrounding the box was amended thoroughly with good-quality compost. Secondly, flood the entire area surrounding this plant and including this plant with water immediately after planting. I would flood this entire area twice a few days apart before turning it over to an irrigation system.

The usual reason for dry fruit is a lack of applied irrigation water. To water this tree, use at least four drip emitters or a coil of drip tubing about 12 inches from the tree trunk. I would cover the soil with a 3- to 4-inch layer of woodchips applied to a distance at least out to the canopy of the tree.

It is very important that the tree receives water deeply and frequently enough to keep the soil moist while the fruit is getting larger. This can be deceiving with such a large tree as the weather. A soil moisture meter inserted into the soil under the mulch about 4 inches deep in three locations will tell you when to irrigate again. You should irrigate when the meter has dropped about halfway, usually when it averages around “5.”

A long, thin piece of rigid steel, like a piece of 4-foot-long rebar, will tell you how many minutes to irrigate. For citrus trees, I would apply enough water to wet the roots to a depth of about 18 inches. Pushing this rebar into the soil after an irrigation will tell you how deeply the water traveled.

If you are locked into a certain number of minutes because you’re irrigating other plants as well, then add enough drip emitters, or increase the size of the emitters, or increase the length of the drip tubing coiled around the tree until enough water is applied with your given number of minutes.

Q: I suddenly lost my favorite dwarf peach tree, Bonanza, over a year ago and I want to replace it. The local nursery said today they probably won’t be carrying Bonanza peach anymore. Could you please advise me where I could purchase one as I don’t know where to look?

A: I just checked online, and Bonanza genetic dwarf or miniature peach tree is available from many nurseries. Check around and see if your nursery will order it for you. If not, you can order it online in November or December from nurseries such as Bay Laurel, Grow Organic, Willis Orchards and others. They will send it to you bare root and in good condition for immediate planting.

I think you will like the fruit from any of the other genetic dwarf peach trees besides Bonanza if you find it locally. When I have tested many of the fruit coming from these genetic dwarf trees it has been good. These include Bonanza II, Garden Gold, Garden Sun, Honey Babe, Pix Zee, Eldorado and many others. Oftentimes retail nurseries don’t know what will become available later in the year, so keep checking.

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.