Posts Tagged: drought
Recently, an outbreak of shoot and twig dieback disease of citrus has been occurring in the main citrus growing regions of the Central Valley of California (Fig 1). The causal agents of this disease were identified as species of Colletotrichum, which are well-known pathogens of citrus and other crops causing anthracnose diseases. At this time, it is unclear how wide-spread the disease is in California citrus orchards, but surveys are being conducted to evaluate the spread of this disease in orchards.
The disease was first noticed in 2012 by several growers and nurserymen in various orchards in the Central Valley. Symptoms included leaf chlorosis, crown thinning, gumming on twigs and shoot dieback, and in severe cases, branch dieback of trees (Fig.2). The most characteristic symptoms of this disease are the gum pockets which appear on young shoots either alone or in clusters and the dieback of twigs and shoots (Fig.3). These symptoms were primarily reported from clementine, mandarin, and navel orange varieties. In order to determine the main cause of this disease, field surveys were conducted in several orchards throughout the Central Valley. Isolations from symptomatic plant samples frequently yielded Colletotrichum species.
Field observations indicate that symptoms initially appear during the early summer months and continue to express until the early fall. Trees showing dieback and gumming symptoms characteristic of this disease are usually sporadic within an orchard and generally only a few twigs or shoots are affected within a tree. Morphological and molecular phylogenetic studies allowed the identification of two distinct species of Colletotrichum (Colletotrichum karstii and Colletotrichum gloeosporioides) associated with twig and shoot dieback. Interestingly, these Colletotrichum species were also isolated from cankers in larger branches. Although C. gloeosporioides is known to cause anthracnose on citrus, a post-harvest disease causing fruit decay, it has not been reported to cause shoot dieback of citrus. C. karstii however has not been reported previously from citrus in California and our laboratory is currently conducting field and green house studies to determine the pathogenicity of this species in citrus.
At present, it is unclear how widespread this disease is in California orchards or how many citrus varieties are susceptible to this disease. Pest control advisors are advised to remain alert and monitor citrus trees for the presence of the disease in the Central Valley (particularly clementine, mandarin, and navel varieties) during the early summer months. Continuing research lead by Dr. Akif Eskalen (UC Riverside) in collaboration with Dr. Florent Trouillas (Kearney Agricultural Research and Extension Center), Dr. Greg Douhan (UCCE Farm Advisor Tulare County), and Craig Kallsen (UCCE Farm Advisor in Kern County) is focused on further understanding the biology of the fungal pathogens as well as factors influencing disease expression in order to develop management strategies against this emerging disease.
Shoot dieback symptoms on Clementine
Branch dieback symptoms on Clementine
Gumming symptoms on Clementine
(photos: A. Eskalen)
California's years-long drought is easing up, with storms delivering rain and snow that has exceeded "normal" for the state, reported Jed Kim for the Marketplace Sustainability Desk. Kim interviewed Dan Sumner, the director of the UC Agricultural Issues Center, a UC Agriculture and Natural Resources statewide program that focuses on such topics such international markets, invasive pests and diseases, and rural development.
Sumner shared a message of hope during the two-minute Marketplace clip.
"So far at least, things are close enough to normal that farmers aren't going to make drastic changes in either their planting decisions or their irrigation decisions," Sumner said.
The abundant rainfall this year will ease pressure on the state's groundwater reservoirs, which have been tapped extensively during the drought to keep crops alive when surface water was unavailable.
"What that does is give us a little cushion in terms of planning for long-term changes," Sumner said.
Kim said the state may need to plan for a future with more limited water resources, "a future that may come sooner rather than later."
The California rainy season is off to a good start, raising hopes that the ongoing drought will be snapped, reported Aaron Davis in the East Bay Times.
"We've seen a sigh of relief from a lot of growers that are right at about half of their total seasonal average and we are halfway through the season," said Paul Verdegaal, UC Cooperative Extension viticulture advisor in San Joaquin County.
The rain is helping flush salts away from the grapevines' rootzones and refill the aquifer, which has been depleted in some areas due to the years-long drought.
The National Weather Service's Seasonal Drought Outlook shows areas of Northern California already free from drought, some areas where the drought designation remains, but is improved, and areas where drought designation removal is "likely."
Half of the state's annual rainfall comes in December, January and February. "This is only mid-December .... So we still have a ways to go in our wet season and Northern California is well above average," said Jeanine Jones, deputy drought manager with the California State Department of Water Resources.
redcThe hills have been dry for a long time, and the long dry fall is bringing animals into the avocado groves that normally stay out in the hills. They want the green cambium of trees and the moisture it provides. And especially rodents will have a field day in the well-maintained orchards.
Gophers are not usually a problem in mature avocados. They will often chew the bark below ground. Ground squirrels when they get hungry can go after just about any part of the tree, trunk branches and fruit. Voles or meadow mice will go after bark about 2 inches above ground. Rabbits can take out bark up to a foot above ground. This bark damage often leads to the yellowing of the canopy and if damage is extensive enough, wilting of the canopy. Growers don't normally see the damage until they see the wilted tree and start looking for the problem. If healing along the margins of the damage is occurring, it means it was damage that was done previously. If margins are still ragged, it means the beasts are still enjoying the tree. Trapping, poisons and a busy Jack Russell terrier are all effective, especially if used together. Voles especially like mulch around the base of the tree, and should be pulled away a foot to 18 inches. They make tunnels thought the mulch which then becomes a diagnostic for identifying the cause.
It's not just avocados that are ravaged by these animals. Citrus is like candy to them and then there's all those acres and acres of almonds that have been planted.
Avocado canopy collapse. Why? Check the tree out. See the red squirrel feeding stations in the background?
Damage that is healing over.
Pottinger asked Ted Grantham, UC Cooperative Extension specialist in the Department of Environmental Science Policy and Management at UC Berkeley, whether the state's fish are adapted to periodic droughts.
Drought is one stressor, he said, but there are additional factors imperiling fish.
"California's native fish have been in steady decline for at least 50 years — in part due to dams, habitat degradation, and the introduction of non-native species," Grantham said. "Native fishes have developed several strategies to cope, but key to their long-term survival is their ability to recover from drought during wet years."
Grantham said there are at least three strategies that would help better manage the state's native fish.
- Better define the amount of water needed to sustain healthy fish populations.
- Create an accurate accounting system for tracking water availability and use.
- Recognize that not all streams are created equal. Some streams support more biological diversity.
The ecosystems science researcher said he is optimistic about the future.
"Although the drought has severely affected California's freshwater ecosystems, it also has raised awareness about the need to improve water management and better prepare for climate change," he said.
For more on threats to California native fish, read Identifying gaps in protecting California's native fish in the UC California Institute for Water Resources' blog The Confluence.