- Author: John Roncoroni
There have been several recent questions about dealing with a plant floating in reservoirs. Here's Weed Scientist Emeritus Roncoroni's response to them.
Pacific Mosquitofern, (Azolla filculoides) often referred to as Azolla, is native to California. It is considered a desirable component of natural habitats and an important food source for waterfowl. Azolla is a floating aquatic fern that spreads by stem fragments and spores. Azolla as being up to 5cm (2 inches) long, but can be as small as ¼ to ½ inches.
Azolla is often misidentified as duckweed (Lemnaspp), another common small floating aquatic plant. Azolla is initially green, like duckweed, but soon turns a red or brown color. This coloration is caused by Azolla's association with a nitrogen-fixing cyanobactrium-Anabeana azollae. In this symbiotic relationship, the bacterium gets a safe home while Azolla is provided nitrogen. This relationship actually plays a very important role in rice production in China and other Asian countries where for centuries Azolla has been used as "green manure" to fertilize rice paddies and has increased production as much as 158%.
Azolla becomes a problem when the population gets so high it begins to clog irrigation screens and pumps. It is reported that cattle won't drink from ponds that are covered with Azolla. One benefit of a dense population of Azolla is that it may actually block sunlight form hitting the pond and may keep other weeds and ‘algae' from growing. If Azolla is controlled the increased sunlight may allow these other weeds to become a problem.
Azolla is widespread in natural ponds and can move in irrigation water, or, like many aquatic weeds spread on the feet and feathers of waterfowl. How does Azolla get from a desirable component of aquatic habitats to seemingly cover the whole pond overnight? Examine the source of water for your pond. If nutrients are introduced into the pond through runoff, or erosion the Azolla population may appear to ‘explode overnight'. Limiting the introduction of these nutrients may help keep the Azolla population at a more acceptable level.
How do you control an existing Azolla problem? Spraying Azolla with herbicides will have only limited success because each of the very small, plant must be treated. If you are successful in controlling a large infestation of Azolla serious degradation of water quality has been reported because of the large mass of rotting dead Azolla decomposing in the pond.
If you check on the web you will see that there is a weevil for sale that is supposed to control Azolla.The only problem for us is that this weevil (Stenopelmus rufinasus) is native to here! It has not been very effective in recent years. USDA researchers in Davis and Albany have been tracking the weevil. Temperature or other environmental factors may play a large part in the success of the weevil and the Azolla and its weevil may eventually regain their natural balance.
What do you do until then? The best recommendation I can give is to drag or rake the Azolla out of your pond. I have heard that some people will use a leaf blower to gather the Azolla in one corner of pond to make it easier to rake. What do you do with the Azolla once you rake it up? It is reported that Azolla makes an excellent ingredient in compost because of its nitrogen content.
Get the scoop on Azolla from "Weed Control in Natural Areas in the Western US"
- Author: Fatemeh Khodadadi
- Author: Ben Faber
Unlike other plants, avocado (Persea americana Mill.), is not a host for very many viral diseases. The occurrence of viral and viroid diseases in avocado is limited to two viroids: Avocado Sunblotch Viroid (ASBV), the smallest known viroid (246-251 nt) belonging to the Avsunviroidae and Potato Spindle Tuber Viroid (PSTV) belonging to the Pospiviroidae. The first, ASBV, only infects avocado and is economically very important and damaging to the avocado industry. While the latter has a wider range of plant hosts and is much more important for vegetables and ornamentals. ASBVd symptoms have been documented in California since the early 1900's and the name of sunblotch was given to this disease because symptoms resemble those of sunburn. Despite the first description of sunblotch in early twentieth century in California, distribution and economic impact of ASBVd on California avocado industry is not well-known yet. Currently, the disease is well-distributed in most of the avocado growing regions in the world and where it impacts yield and fruit quality. The viroid is unevenly distributed in the tree and sometimes the infected tree does not show symptoms. Symptomless trees play a significant role in the epidemiology and latent spread of the pathogen. The full impact of ASBVd is difficult to estimate because of the symptomless nature of the disease; however, researchers have documented known symptomatic and asymptomatic ‘Hass' and found up to 75% and 30% yield reduction, respectively. Additionally, ASBVd symptoms may vary under the influence of different environment, avocado cultivar, and variant of the viroid. viroid is a stable molecule which can survive outside the host cell environment for almost 7 weeks, ASBV has been documented to remain viable for 3 months in bee pollen taken from beehives.
ASBVd causes symptoms on all tree parts including fruit, leaves, branches, and twigs. While symptoms on leaves e.g., discoloration and physical distortion may rarely be seen, fruit typically show depressed/sunken scars in white, yellow, and red color which are dominantly seen at the pedicle end. Sometimes severely infected fruit show necrotic areas at the center of the crevices. Contradictory reports exist regarding the impact of ASBVd on yield, some reports state that infected trees produce abundant but small fruit in size while in some, tree yield was reported to be severely reduced. Small and misshaped fruit resulting from the viroid impact fruit marketing value. Alligator skin/bark is another typical symptom of ASBVd which is described as rectangular cracking appearance on bark of large branches and trunk of old trees. Infected trees may look stunted and old trees grow into a low and flattened shape with branches bowing toward the ground.
Future Explorations. The pace of the global avocado production is remarkedly high compared to other tropical fruits. Such increase in production is driven by high consumer demand and requires expansion of the industry and may involve the exchange of plant materials. To meet this goal, we first need to optimize reliable, sensitive, fast, and, if possible, affordable techniques to detect the pathogen. However, detection is challenging due to the uneven and irregular distribution of the viroid in avocado trees as well as the presence of asymptomatic trees.
Another interesting and challenging point that will require the attention of researchers is that some symptomatic trees spontaneously become asymptomatic and vice versa. It is assumed that stress plays a part in symptomology, but more research is needed to determine this.
Observations show that all avocado cultivars are susceptible to ASBVd but more information about the susceptibility of rootstocks and their roles in root-to-root transmission or combination of rootstock-scions is needed to be explored. Our next steps on the status of this disease will be focused on understanding the distribution and economic impacts, developing fast and reliable detection techniques with ability to detect the pathogen in any infected part of the tree, and the possible variants associated with the different symptoms. Lastly, it would be ideal if we could develop methods to control the disease.
- Author: Ben Faber
Implications for tequila growers
Agave are a common sight in Southern California and are frequently used in landscaping for homes, businesses, and in public spaces. You've almost certainly seen agave growing in your neighborhood and may even have some growing yourself. Many varieties of agave are grown in San Diego nurseries to keep pace with the demand for these plants across the state and country. As the climate continues to warm and California becomes increasingly dry, hardy and water-conscious plants like agave will more frequently be used in xeriscaping and as ornamentals. However, there is an almost invisible enemy that threatens many of these agave plants. Greasy streaks and smudges appear on leaves, followed by lesions and plant decline. Sometimes, the entire core of the agave collapses. The plants look sickly and unattractive, dismaying homeowners and nursery growers alike. What is causing this damage? Mites.
Agave mites are a difficult pest to manage and can be a serious problem on multiple types of agave. Being proactive and removing infected plants is currently the best way to protect your agave from mite infestations. Recognizing agave mite symptoms and being ruthless with eliminating plants is key to preventing damage. Research on management options is just beginning and there are still many unknowns, so please contact us if you are having issues with agave mites at your nursery, if you have questions, or if you think there is something we should know about. Stayed tuned for future results and hopefully some better news on how to manage this tiny menace!
- Author: Ben Faber
Avocado fruit set is lousy up and down the coast this year. The trees were flowering in many places from January to June. They kept trying to set fruit, but nada. According to Daniel Swain at UCLA Weather West, this was the coldest winter on record without freeze and a long drizzly spring with little sun - cool days and night time temperatures. Cold day time temps mean poor pollinator activity. Honeybees just weren't working much. Then if pollen is transferred to the female stage flower, it takes 48 hours of above 50 deg F for fertilization - the deposition of pollen on the stigma and growth of the pollen tube down the style to the ovule. If the temp at night drops below 50, the process stops and the ovule never gets fertilized. So it doesn't matter if day time is warm and the pollinators are working, if it gets cold at night, the fertilization process stops.
So that's exactly what happened. Cool days for poor pollination and then cold nights for fertilization. It didn't matter how hard the tree flowered, it just wasn't the right weather. It didn't matter if you sprayed gibberellic acid. The weather was just not right for fruit set this tear.
And this should set off alternate bearing for next year if we have the right weather conditions. So you might think seriously about pruning because there could be a branch breaking crop set next spring – if the weather is right.
Interestingly,just as an observation it looks like GEM and Lamb Hass right next to Hass did set fruit just at the end of the flowering period. So there is some difference in varieties to weather conditions.
Image from American Museum of Natural History
The cool/moist weather has also created conditions for anthracnose and alternaria in fruit which causes problems for the consumer, but also caused early fruit drop and color change in the fruit on the tree. The trees just can't hold on to the fruit as one normally expects. It's a tough year for avocado growers. In the Central Valley all the stone fruit is late by almost a month. Apricots and cherries are just coming in season when they should have been available in late May.
Weird and Maybe we are going to have to get used to it.
Read more about the Basics of flowering
pollination - movement of male pollen to female stigma
fertilization - growth of pollen tube down to stimulate ovule
- Author: Ben Faber
It seems like the simplest thing is the hardest. Recently, I was called out to evaluate why newly planted trees were failing at two sites and they both had a common problem. In one case, the trees had been planted too deeply at the beginning. At another, a large amount of planting amendment had been incorporated, and over a year's time, the trees had settled, so that they too had their graft unions covered with soil. In the latter case, the trees' unions were 4-8 inches below grade. It seems appropriate to review basic planting practices. In the best-case scenario, trees are planted from February to May, but depending on the area, they can be planted at other times, as well. So, I just got a call about planting, so it's probably time for a refresher.
Often times the grower harkens to the old adage: “dig a $5 hole for a 50-cent plant.” And so a lot of time and money and energy are put into that hole. Nothing costs $5 anymore. Trees cost closer to $50, so there might be a greater urge to do it all right. So the first thing first is forget the planting mix and those mycorrhizal inoculums. They either don't work or they might just damage your expensive tree.
Adding organic matter to a planting hole appears to be a promising step towards achieving that five-dollar hole. It seems logical that steer manure, peat moss, compost, etc. would improve poor soils by increasing aeration, nutritional value, and water holding capacity. And it does - in the immediate vicinity of the planting hole. Eventually, amended planting holes will have negative consequences to plant health.
The initial results are positive; roots grow vigorously in this ideal environment as long as irrigation is provided. But what happens when these roots encounter the interface between the planting hole and the native soil? Native soil contains fewer available nutrients, is more finely textured and is less aerated. Roots react much in the same way as they do in containers: they circle the edge of the interface and grow back into that more hospitable environment of the planting hole. The roots do not establish in the native soil, eventually resulting in reduced growth rates.
Soil water movement is problematic as well. Amended backfill has markedly different characteristics than surrounding native soil; it is more porous and water will wick away to the finer-textured native soil. In the summer, moisture within the planting hole will be depleted by the plant but not replaced by water held more tightly in the native soil. When irrigating, water will move quickly through the amended soil only to be held back by the more slowly draining native soil. The resulting bathtub effect, where water accumulates in the planting hole, floods the roots and eventually kills the plant.
Finally, all organic material eventually decomposes. If you've incorporated organic matter, within a few years that organic matter will have become ash and the bulk has turned into carbon dioxide. The soil will have settled and if you have followed the rule of thumb of 25% by volume, the trunk stem probably will have sunk below grade and the bud union will be buried. The buried union only exacerbates the flooding problem during irrigation or wet conditions. This was the problem that I went out to see.
Basic planting steps
1) Dig a hole somewhat wider, but no deeper than the sleeve that the tree comes in. Making the hole wider (18 inches) allows room to manipulate the tree by hand and remove the sleeve once it is in the hole. Making the hole deeper than the sleeve allows for soil to accumulate around the graft union. Even if the hole is backfilled to the “appropriate” depth, because of subsidence of the loose earth, the tree can become buried. Do not put gravel in the bottom of the hole. This is commonly thought to improve drainage. It does not, it makes it worse.
2) Gently tamp loose earth around the tree. Do not back fill with a planting mix. This creates a textural discontinuity which interferes with water movement both to and anyway from the tree. The fill soil should be free of clods to avoid air gaps and poor contact between roots and soil. Do not cover the root ball with soil; the irrigation water needs to come into direct contact with the root ball.
3) The trees should be watered as soon as is practical after planting.
4) Using drip irrigation the, the emitter should be near the trunk, so that water goes directly onto the root ball. Shrinking and swelling of the polyethylene tubing can move the emitter off the ball.
5) After about 4-6 months the drip emitters can be moved from the trunk to 6-8 inches from the tree. Moving the emitters avoids keeping the trunks wet and reduces the likelihood of crown rot.
6) In most situations, newly planted trees should be irrigated every 5-10 days with 2-5 gallons of water for the first 2-4 months until the roots get out into the bulk soil. Depending on what the weather is like, they still might require frequent irrigations, because the rooted volume holding water is still small. After the first year in the ground, another dripper can be installed on the opposite side of the tree. As the tree grows the number of drippers should be increased or the system converted to fan or microsprinklers.
And in the case of mycorrhizae, they are wonderful. They are nature's gift to all of us. They aid plants in their uptake of nutrients, improve plant health and may actively transmit information from one plant to another. They are a diverse range of fungi associated with plant roots and are everywhere – even Antarctica. And that's the point, introducing them to the planting is not going to help. They are there already, a sea of them. Putting a few nursery-grown spores into a planting hole when there are already highly adapted fungi present just does not happen easily. So not using a planting mix and not adding mycorrhizal inoculum is going to make that hole a little bit cheaper.
In root rot conditions
Planting in ground that has had root rot can add some new steps to the planting process. On relatively flat ground (
The key to root rot has always been dependent on irrigation management. There is nothing more important than getting the right amount on at the right time. If you are doing interplanting into an existing orchard where trees have died, it is imperative that the new trees be put on their separate irrigation line so that they can be irrigated according to their needs. Simply putting a smaller emitter on the young trees compared to the older trees means that they will still be irrigated on a cycle that is not optimum for their survival. It doesn't matter if you are using clonals; they will die just as easily with poor water management as a seedling.
Check out the pictorial guide to proper tree planting