One key to maintaining a productive and profitable almond orchard is being able to recognize and manage trunk and scaffold diseases. Three diseases are of particular concern, says David Doll, a University of California Cooperative Extension farm adviser in Merced. These are botryosphaeria canker, which is rapidly emerging as a serious problem; hull rot, which causes the greatest yield loss; and ceratocystis canker, the most common, but also the most overlooked. Here’s a brief look at each.
1. Botryosphaeria Canker
This fungal disease, also known as band canker, used to be somewhat rare. But now growers are seeing it in both natural openings and pruning wounds in the scaffolds, especially after a rainfall. Spores, which are spread through the air, can come from a wide variety of hosts, as the pathogen is found in many trees in both agricultural and natural settings.
The Padre variety is very susceptible to botryosphaeria, especially when it is growing with high vigor. Incidence in other varieties has been limited. Most commonly, it has been seen in over-fertilized young trees (first through fourth leaf) that are being pushed hard for early production. That is why Doll says it’s being seen more and more. To avoid infection, it’s important not to prune near a rainfall, and avoid large pruning wounds. But if a tree does get band canker, it must be removed. “And when you’re removing it, you must remove the whole tree,” says Doll, “even the stump.”
2. Hull Rot
This is of particular concern this time of year — at hull split. Trees that are over-irrigated and/or over-fertilized are most susceptible to this windborne fungal disease, says Doll. “Dr. Beth Teviotdale (retired Extension specialist) called hull rot the gout of almonds,” he says, “because it can be caused by too much food and water.”
Hull rot is also a big problem because the industry’s most prized variety, the Nonpareil, is one of the most susceptible varieties. To keep your Nonpareils and others from getting it, Doll recommends using a pressure chamber to check your irrigation level this time of year. A fully irrigated orchard will be at minus 10 bars or so, and growers should shoot for a reading of minus 15 bars. “You can help control it through slight water stress at the onset of hull split,” he says. “Studies show that when you induce deficit irrigation, the incidence of hull rot is definitely reduced.”
3. Ceratocystis Canker
Also known as “shaker’s disease” because it is often caused by injuries that occur from a shaker at harvest, this fungal disease is transferred around the orchard by fruit flies and beetles, which are attracted to a tree’s wound. Ceratocystis canker reduces yield by causing scaffold loss. It is the most common disease of the trunk and scaffold, and can significantly shorten the life of an orchard, says Doll. “It can be a big problem because an orchard will look beautiful from the road,” he says, “but then when you walk through it you see that every branch has some level of ceratocystis.”
Improper harvesting, whether due to bad shaker injury, or shaking too soon after a rainfall or irrigation, are the biggest causes, says Doll. Letting the tree dry down after a rainfall will take care of the latter, while using a good quality shaker will help with the former. “The new shakers will reduce damage to the tree because the pads and the heads are not as rough on the trees,” he says. “I think new technology — as well as the increase in skilled operators — will reduce the problem to a great degree.”
Think Before You Prune
Several times in the past few years, American/Western Fruit Grower has published articles presenting data from researchers that questions the value of pruning almond trees. The trials centered on the main reason growers prune, to increase yields. But in trials up and down the state, University of California Cooperative Extension (UCCE) farm advisers have not found any evidence that pruning increases yields. It’s still somewhat controversial, because it goes against the traditional cultural practice that all deciduous fruit and nut trees should be pruned.
If all the yield data isn’t enough to make almond growers question whether to prune, David Doll, a UCCE farm adviser in Merced, has found that pruning can lead to disease problems. Several pathogenic fungi of almond, including ceratocystis, botryosphaeria, and phytophthora, have caused cankers after gaining entrance through pruning wounds. Doll cautions growers to keep in mind that any time you injure the tree’s cambium layer — such as with a pruning cut — you’re opening up the tree to potential disease problems.
In general, he says that growers should avoid disturbing the cambium layer when the chance of infection is high, which is before or after a rainfall. Because of that, if growers feel they must prune after harvest, they should do so in October or early November, before the state’s rainy season begins. Doll says that he’s finding more and more growers are moving away from pruning, anyway, and he’s not surprised. “Not only do you save on labor, but there’s less disease incidence as well,” he says. “Why waste the money?”