New technique has potential
to protect citrus from HLB
Citrus greening, also called Huanglongbing (HLB), is devastating the citrus industry. Florida alone has experienced a 50 to 75 percent reduction in citrus production. There are no resistant varieties of citrus available and limited disease control measures.
Some scientists think it is possible that orange juice could one day become as expensive and rare as caviar. In an effort to prevent this, three plant pathologists at the University of California-Berkeley and United States Department of Agriculture conducted research into ways to boost citrus immunity and protect the valuable fruit against citrus greening.
Because the bacteria that causes citrus greening cannot be grown in a lab, scientists have to find novel ways to conduct experiments. The University of California-Berkeley/USDA team looked at many different strains of the bacteria that cause citrus greening to see if they could identify peptides (a compound of two or more amino acids) that would trigger immune responses.
"This was a long list, so we narrowed it down by selecting small peptides that were a bit different in their peptide sequence, which might imply that the bacterium had made those sequence changes so that they wouldn't be recognized by the plant immune system," explained Jennifer D. Lewis, group leader of the research team. "Then we further narrowed that list to peptides from strains that caused disease in citrus."
Through this research, they showed that two peptides could trigger immune responses in multiple plant species, including citrus. These peptides may play a role in preventing or reducing yield loss from citrus greening.
According to Lewis, "We thought it was particularly interesting that some of the peptides predicted to elicit a response, could actually trigger immune responses in multiple plant species. This suggests that the immune response to these peptides is conserved across species."
There has been a good overall discussion of herbicide resistance found in plants and how they can affect orchard management. Check out this presentation by UC Cooperative Extension Weedologist, Brad Hanson, in the "past Webinars" section:
And read more about glyphosate resistance in orchards:
UC Ag Experts Talk:
Managing Glyphosate-Resistant Weeds in Orchard Crops
Description: One hour webinar about glyphosate-resistant weed management in orchards, delivered by Dr. Brad Hanson. One CEU (other) from the DPR is approved.
Time: Apr 24, 2019 3:00 PM in Pacific Time (US and Canada)
Recorded version will be published on UC IPM YouTube channel about a week after the webinar.
The link to register is https://ucanr.zoom.us/webinar/register/WN_96wd2GBMQl2Ou4i4oSwTTg
More information about the webinar series UC Ag Experts talk: https://ucanr.edu/sites/ucexpertstalk/
- Author: Lynn M. Sosnoskie, PhD
UCCE Agronomy and Weed Science Advisor, Merced and Madera Counties
Weeds compete with crops for light, water, and nutrients, which can result in yield reductions. Weeds can also interfere with crop production by serving as alternate hosts for pests and pathogens, providing habitat for rodents, and impeding harvest operations, among other impacts. Natural areas can also be impacted by weed species when they reduce aesthetics and disrupt ecosystem services. As a consequence, growers and land managers employ a variety of control strategies, including the application of herbicides, to manage unwanted vegetation.
Although herbicides can be effective tools for controlling undesirable plants, failures can and do occur. Weeds may escape chemical treatments for several reasons including: the selection of an ineffective herbicide or herbicide rate, improperly calibrated or malfunctioning equipment, applications made at a time when the target species is not susceptible to control, the use of herbicides under adverse environmental conditions, and the evolution of herbicide resistance.
As of 3 January 2019, there are 496 confirmed cases (species x site of action) of herbicide resistance, worldwide. Current reports provided by the International Survey of Herbicide Resistant Weeds (www.weedscience.org) indicate that 255 different species (148 dicots and 107 monocots) have evolved resistance to 163 different herbicides across 23 of 26 known sites of action. With respect to the United States, 161 unique instances of resistance have been documented. Most resistances (52 cases) are to the acetolactate synthase (ALS) inhibitors followed by the photosystem II (PS II) inhibitors (26 cases), 5-enol-pyruvyl-shikimate-3-phosphate synthase (EPSPS) inhibitors (17 cases), and the acetyl-CoA carboxylase (ACCase) inhibitors (15 cases).
Currently, in California, there are 30 confirmed occurrences of herbicide resistance. Twenty-four of those cases are to a single site of action (Table 1). The most frequently encountered resistances have been to the ALS and EPSPS inhibitors (7 each). Five weed species (late watergrass (Echinochloa oryzicola), barnyardgrass (Echinochloa crus-galli ssp. crus-galli), hairy fleabane (Conyza bonariensis), horseweed (Conyza canadensis), and Italian ryegrass (Lolium perenne ssp. multiflorum)) have populations with documented resistance to up to four herbicide sites of action (Table 2).
Growers and land managers can take several actions to thwart the evolution and spread of herbicide resistant weeds. First and foremost is scouting fields following herbicide applications and keeping careful records of herbicide performance to quickly identify repeated instances of weed control failure. Pesticide applicators should ensure that their equipment is properly calibrated and that they are applying effective herbicides at appropriate rates to manage the target species. Whenever possible, diversify herbicides to reduce chemical selection pressure. If appropriate, incorporate physical and cultural weed control practices into a vegetation management plan. Be sure to control unwanted plants when they are small and never allow escapes to set seed. Clean equipment to prevent seeds of herbicide-resistant weed species from moving between infested and non-infested sites and don't forget that unmanaged roadsides, canal banks, fence lines, and field margins, etc., can serve as a source of propagules.
Table 1. A summary of herbicide resistance in California to single sites of action.
Table 2. Weed species in California with confirmed resistance to multiple herbicide sites of action
A recent call about the poor control of marestail (horseweed, Conyza canadensis) to glyphosate (Roundup®) wasn't surprising, but that paraquat didnt do the trick was. It turns out that there is multiple resistance to the materials. If horseweed is resistant to glyphosate it is possibly going to be resistant to paraguat which also means that hairy fleabane which has glyphosate resistance could also show resistance to paraquat. A recent study reports on the increased Conyza resistance to paraquat (Distribution of Conyza sp. in Orchards of California and Response to Glyphosate and Paraquat, Moretti et al, https://doi.org/10.1614/WS-D-15-00174.1):
Resistance to glyphosate in hairy fleabane and horseweed is a problem in orchards and vineyards in California. Population genetic analyses suggest that glyphosate resistance evolved multiple times in both species, but it is unknown if resistance to other herbicides is also present. Two approaches of research were undertaken to further evaluate herbicide resistance in Conyza sp. in the perennial crop systems of California. In the initial study, the distribution of Conyza sp. in the Central Valley, using a semistructured field survey, was coupled with evaluation of the presence and level of glyphosate resistance in plants grown from field-collected seed. In a subsequent study, single-seed descendants representing distinct genetic groups were self-pollinated in the greenhouse and these accessions were evaluated for response to glyphosate and paraquat. Conyza sp. were commonly found throughout the Central Valley and glyphosate-resistant individuals were confirmed in all field collections of both species. The level of glyphosate resistance among field collections varied from 5- to 21-fold compared with 50% glyphosate resistance (GR50) of the susceptible, with exception of one region with a GR50 similar to the susceptible. When self-pollinated accessions from different genetic groups were screened, the level of glyphosate resistance, on the basis of GR50 values, ranged from 1.7- to 42.5-fold in hairy fleabane, and 5.9- to 40.3-fold in horseweed. Three accessions of hairy fleabane from different genetic groups were also resistant to paraquat (40.1- to 352.5-fold). One glyphosate-resistant horseweed accession was resistant to paraquat (322.8-fold), which is the first confirmed case in California. All paraquat-resistant accessions of Conyza sp. identified so far have also been resistant to glyphosate, probably because glyphosate resistance is already widespread in the state. Because glyphosate and paraquat resistances are found across a wide geographical range and in accessions from distinct genetic groups, multiple resistant Conyza sp. likely developed independently several times in California.