- Author: Belinda Messenger-Sikes
After years of drought, we welcome rain in California. But we also recognize that rain can help spread a number of plant diseases. Rain and wind can splash bacteria and fungi from infected leaves, branches, and blossoms to uninfected parts of the tree. The fungal diseases anthracnose, peach leaf curl, scab, shot hole blight, and the bacterial disease fire blight can all be spread by rain splash. This ability to spread by water makes these diseases more common after a wet spring. With 2023 bringing quite a bit of rainfall and 2024 looking similarly wet, we want to focus on some common rain-dispersed diseases.
Anthracnose
Anthracnose affects many trees including almond, citrus, Chinese elm, and ash (Figure 1). In the spring, fungi produce spores on leaves and twigs that can be spread to new growth via water splashing. Under prolonged wet conditions, this cycle of spore production and spread can occur repeatedly. Anthracnose can defoliate trees, although it is not a lethal disease in most of its hosts. Once symptoms develop or become severe in a growing season, anthracnose can't be effectively controlled. It's best to rely on prevention of the disease by planting resistant varieties. For more information about identification and management of anthracnose, see Pest Notes: Anthracnose.
Fire blight
Pome fruit trees like pear and apple, and other related trees like pyracantha are often affected by fire blight. This destructive disease spreads in the early spring when rain splashes water from infected leaves and blossoms to healthy plant parts. Pollinators like bees can also spread the disease as they visit blossoms. Symptoms may not be noticeable until later when shoots and flowers shrivel and blacken. New growth is especially vulnerable to infection, so avoid heavy fertilization and pruning during mild wet weather.
The Pest Notes: Fire Blight contains much more detail on identification and management of this disease.
Peach leaf curl
Many home gardeners are familiar with the distorted, reddish leaves (Figure 2) caused by peach leaf curl, a disease that affects peach and nectarine trees. The fruit can also be damaged, becoming corky and cracked. Cool, wet weather favors the spread of the disease, but treatment must be started before the spring rains. Preventative sprays should occur after leaf drop but before flower buds swell. Resistant varieties are available to prevent this disease from occurring. Pest Notes: Peach Leaf Curl provides details about this disease and its treatment.
Scab
Various fungi cause the disease scab, which appears as spots and scabby blemishes on fruit and leaves. Apple scab is especially serious during wet springs and in the cool moist coastal areas of California, although these conditions are also conducive for development of other scab diseases. This disease can ruin the harvest of commercially grown apples and pears but in home gardens, some damage can be tolerated. Limit both apple and pear scab on backyard trees by removing and composting fallen leaves and fruit in the fall.
Information specifically about apple and pear scab can be found in the Pest Notes: Apple and Pear Scab. For more about other scab diseases, see http://ipm.ucanr.edu/PMG/GARDEN/PLANTS/DISEASES/scab.html.
Shot hole blight
Shot hole blight, also known as Coryneum blight, is caused by the fungus Wilsonomyces carpophilus. It affects almonds, apricots, and other Prunus species, as well as English laurel. Symptoms appear in the spring as small reddish spots that turn brown and drop out, leaving holes in the leaves (Figure 3). Fruit and twigs can also be affected. Since this disease is worsened by continuous leaf wetting, avoid irrigating foliage. For more information, see the UC IPM page on shot hole blight at http://ipm.ucanr.edu/PMG/GARDEN/FRUIT/DISEASE/shothole.html.
While you can't control the rain from spreading these diseases, good plant care and sanitation practices can help limit the severity. See the publications above as well as other pages on the UC IPM website to find out more.
[Originally featured in the Spring 2024 edition of the Home & Garden Pest Newsletter]
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- Author: Ben Faber
A repeat post from the recent past
- Author: Ben Faber
What Can Happen With Too Much Rain? Watch that Mulch!!!
Rain is wonderful stuff. If it comes and washes the accumulated salts of the last several years out of the root zones of citrus and avocado, that's a good thing. But what happens if there is a little too much of the good stuff? In the winter of 2005, Venture got over 40” of rain, which is 200% of what is normal. The last time big rains occurred prior to that was in the winter of 1997-98. That year the rains were evenly spaced on almost a weekly basis through the winter and into the late spring and over 50" fell. That year we had major problems with both citrus and avocados collapsing from asphyxiation. The same occurred in 2005, but not so pronounced.
Rain is good, right?
In 2003 we had a lot more rain than we normally see and in Carpinteria it rained 4 inches in July!!!!!! In some young trees with poorly developed root systems, we have seen some collapse from asphyxiation. Avocados tend to be more susceptible than citrus, and some rootstocks more than others. Even big trees collapsed.
Asphyxiation is a physiological problem that may affect certain branches, whole limbs or the entire tree. Leaves wilt and may fall, the fruit withers and drops and the branches die back to a greater or lesser extent. The condition develops so rapidly that it may be regarded as a form of collapse. Usually, the larger stems and branches remain alive, and after a time, vigorous new growth is put out so that the tree tends to recover. Young trees can be harder hit, but sunburn damage from lack of leaves may be more of a problem.
Asphyxiation is related to the air and water conditions of the soil. The trouble appears mainly in fine-textured or shallow soils with impervious sub-soils. In 1997-98, this even occurred on slopes with normally good drainage because the rains were so frequent. When such soils are over-irrigated or wetted by rains, the water displaces the soil oxygen. The smaller roots die when deprived of oxygen. When the stress of water shortage develops, the impaired roots are unable to supply water to the leaves rapidly enough and the tree collapses. The condition is accentuated when rainy weather is followed by winds or warm conditions.
It doesn't take standing water to have asphyxiation occur.
- Author: Elinor Teague
Many varieties of citrus and other fruit trees have set a bumper crop of small fruit this season. Alternate bearing or the setting of fewer fruit every other year or two is normal for fruiting trees, but this year's bounty of mini fruit is unusual. The cause could be last season's long, cool, wet spring weather that delayed blossom set and pollination. Or it could be that the immature heavy crop of fruit should have been thinned early. The fruit is juicy and sweet. Just remember that the best way to store citrus fruit is to leave it on the tree until it falls off.
Rose leaves are still bright green and firmly attached to most bushes. Roses will set flowers all winter long if temperatures don't drop to freezing levels. In the Central Valley with its mild, short winters, rose lovers usually strip browning leaves from their bushes in mid-November to force dormancy by using water from a hose to blast off the dead and dying leaves. But you might need to hand strip green leaves from roses this winter. Do it after you've pruned to minimize the work.
Weed seeds germinate quickly when the soil temperature warms above 50 degrees. That often happens during winter warm spells like those we had during December. As you are using a wiggle hoe to cut down tiny weeds, you'll notice that it's much easier to till of the soil in areas that have been kept covered with a three- to four-inch layer of mulch. Those of us who have begun spreading the fall leaves into planting beds instead of raking them up are seeing that the leaf mulch does reduce weed germination, but because we haven't had much rain yet this winter, the dry fall leaves have barely begun to decompose. Rake or blow the leaves to turn them and encourage faster decomposition.
- Author: Ben Faber
What Can Happen With Too Much Rain? Watch that Mulch!!!
Rain is wonderful stuff. If it comes and washes the accumulated salts of the last several years out of the root zones of citrus and avocado, that's a good thing. But what happens if there is a little too much of the good stuff? In the winter of 2005, Venture got over 40” of rain, which is 200% of what is normal. The last time big rains occurred prior to that was in the winter of 1997-98. That year the rains were evenly spaced on almost a weekly basis through the winter and into the late spring and over 50" fell. That year we had major problems with both citrus and avocados collapsing from asphyxiation. The same occurred in 2005, but not so pronounced.
Rain is good, right?
In 2003 we had a lot more rain than we normally see and in Carpinteria it rained 4 inches in July!!!!!! In some young trees with poorly developed root systems, we have seen some collapse from asphyxiation. Avocados tend to be more susceptible than citrus, and some rootstocks more than others. Even big trees collapsed.
Asphyxiation is a physiological problem that may affect certain branches, whole limbs or the entire tree. Leaves wilt and may fall, the fruit withers and drops and the branches die back to a greater or lesser extent. The condition develops so rapidly that it may be regarded as a form of collapse. Usually, the larger stems and branches remain alive, and after a time, vigorous new growth is put out so that the tree tends to recover. Young trees can be harder hit, but sunburn damage from lack of leaves may be more of a problem.
Asphyxiation is related to the air and water conditions of the soil. The trouble appears mainly in fine-textured or shallow soils with impervious sub-soils. In 1997-98, this even occurred on slopes with normally good drainage because the rains were so frequent. When such soils are over-irrigated or wetted by rains, the water displaces the soil oxygen. The smaller roots die when deprived of oxygen. When the stress of water shortage develops, the impaired roots are unable to supply water to the leaves rapidly enough and the tree collapses. The condition is accentuated when rainy weather is followed by winds or warm conditions.
It doesn't take standing water to have asphyxiation occur.
Mulch is good, right?
It can get spread awfully thick
Mulch does lots of good things, like reducing erosion, controlling weeds, improving infiltration, increasing organic matter to control root disease, reducing soil temperatures, reducing evaporation loss and therefore improving soil moisture content. Ah, but that is a problem when there is a lot of rain. It can lead to asphyxiation, just because of that last point. In rainy conditions, there needs to be a period of air entry into the soil and mulch because it reduces evaporative loss can keep the soil TOO wet, leading to asphyxiation.
Asphyxiated tree that has been overmulched and is now recovering after whitewashing.
So what do you do? With a thick layer of mulch when there is too much rain? You pull it away several feet from the base of the tree, so that sun can dry out the soil. It's a lot of work, especially after you have put a lot of work into spreading it.
The roots need to dry out and it can only happen if exposed to light and wind and ambient temperature
On flat ground, your avocados are planted on mounds or berms. And the soil texture is somewhat sandy and well drained. In that case, unless we get rain like 1997-98 when Ojai got nearly 50 inches and it rained just about every week from November through April and it seemed everything was under water, most trees should be all right. But, this last rainfall year I saw cases where unbermed, heavy soil avocado plantings had severe asphyxiation problems. So be forewarned.
- Author: Ben Faber
I recently harvested 'GEM' avocados and found that some of the fruit was splitting. The split fruit was just turning from a bright green to one with dark tinges. There were also gobs of snails and slugs on the fruit. I mentioned the cracking fruit to a grower who had been harvesting ‘Hass' fruit and the comment was that their fruit was turning black on the tree and falling. This is way early for ‘Hass' to show this level of maturity.
Several mandarin growers also mention peel breakdown or rind disorder. There can be brown, water-soaked blotches or dark, sunken areas developing on the rind after rainy weather. The fruit matures rapidly, turning orange early. Secondary fungi can then colonize the affected areas. It's more of a problem in northern California where thin-skinned satsumas are grown and there are early rains. But accelerated maturity and fruit/whole plant breakdown can happen on all citrus and many other fruiting plant species.
This is another example of accelerated maturity due to wet roots. The roots need air, like we do, and when they are asphyxiated for even short periods of time, they start signaling the rest of the plant that something is wrong. Prolonged rainy periods or actual ponding conditions will lead to ethylene precursors that start the maturation process – “banana in a bag” effect. This is in spite of many crops being delayed in their maturity because of the cold winter we have had. Drown the roots, and they start responding as if they are choking. They are. In some cases, leaves will droop (epinasty or wilting) and drop, immediately. And then the fruit may drop right away or then just start rapidly maturing/ripening on the tree. Once this process starts, there's no stopping it. Some growers, seeing fruit drop have gone out to start harvests, but then inadvertently gotten their forklifts stuck in the mud. Rain can have its downsides.