May 18, 2022Weeds and Insect Management in Desert ProduceTo contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Figure 1. The Colorado River watershed that includes seven western U.S. states and 2
Mexican states, supports >40M people, >5.5M acres of farmland, and 22 Native American
tribes. Source: USGS.
Table 1. Drought Contingency Plan reductions in Arizona’s allocation from the Colorado River.
FAS = feet above sea level; KAF = thousand acre-feet; MAF = million acre-feet
Figure 2. General outline of reductions in water allocations by general source associated
with the Drought Contingency Plan.
So, this translates to nearly 480KAF less water moving through the Grand Canyon and into Lake Mead in FY22 and it puts the lower basin (the region below Lake Powell, Figure 1) one step closer to Tier 2 reductions.
The BoR periodically runs a series of model projections regarding the water levels at the dams for both Lakes Mead and Powell. Results from these models recently projected future water levels at the dams for the end of calendar year (CY) 2022 and 2023. For Lake Mead the end of CY 2022 projections include a most probable level of 1,049.37 FAS and the lowest probable level at 1,047.10 FAS. These results project the need for a Tier 2a level declaration by the end of CY 2022.
For the end of 2023, Lake Mead projections include a most probable level 1,035.63 FAS, which would trigger Tier 2b reductions in Colorado River water allocations to Arizona and a most probable minimum level of 1,020.63 FAS, which would necessitate Tier 3 reductions.
Due to the recent changes in river management plans that have been announced by the BoR associated with Lake Powell, these model projections for the end of CY 2022 and 2023 are probably high and probably project an overly optimistic condition. But we can see that the probability of moving into the Tier 2a, 2b, and Tier 3 reductions are very likely to occur by the end of 2023, if not sooner.
The warnings of John Wesley Powell, the famous one-armed Civil War Veteran who first directed an expedition down the Colorado River and the Grand Canyon in 1869, are certainly very prescient in the face of the circumstances we are dealing with today. In an address to the Montana Congressional Convention in 1889 he offered the following statement: “All the great values of this territory have ultimately to be measured to you in acre feet”. That is incredibly prophetic and certainly true in the reality of conditions that we are dealing with on the Colorado River today.
Essentially what we have today on the Colorado River is a supply and demand problem. Very simply, our demand and extractions from the Colorado River have been greater than the supply and what the river can provide. As a result, the great reservoirs on the river system, the savings accounts so to speak, have been depleted. We must come to grips with that reality and decide how the allocations of Colorado River water must be adjusted to bring them into an appropriate balance with the water we do have in the river.
The overall situation with the Colorado River offers some good news and some bad news for all of us depending on this water to live, work, and survive in this desert. The average annual flow of the Colorado River between 2000 and 2018 has been approximately 12.4MAF, which is 16 % lower than the 1906-2017 average of 14.8MAF/year. So, the good news is with the recognition that we have ~ 12MAF average annual flow in the Colorado River under these megadrought conditions. The bad news is that the Colorado River system is budgeted for 16.5MAF of allocated water between the U.S. and Mexico. Thus, there is a functional difference of >4MAF and the fact is that we must reconcile that difference and fast action is needed, much faster than our water governance systems normally operate.
Arizona agricultural is responsible for ~ 70% of the water diversions on the Colorado River and agriculture is taking the reductions now with Tier 1 and will carry much of the responsibility for the Tier 2 reductions. How agriculture fares in this process of Colorado River management in response to the water shortages is critical for the future of Arizona agriculture and the overall complexion of life in the desert Southwest.
I believe agriculture will prevail, but a lot of hard work, difficult decisions, and changes are ahead of us in the near future. Agriculture has some strong and effective groups working in this decision-making arena for water governance on the Colorado River and we need to be sure that practical agricultural considerations are being made in the process with both short and long-term implications.
DISEASE: Center Rot of Onion
PATHOGEN: Pantoea ananatis, Pantoea agglomerans, Pantoea alli and Pantoea stewartii subsp. indologenes
HOSTS: Onion (Allium cepa L.), garlic (Allium sativum L.), shallots (Allium cepa var. aggregatum L.), leeks (Allium ampeloprasum L.), chives (Allium schoenoprasum L.).
Symptoms and signs
Center rot of onion has not been a major problem in the desert southwest but when the environment is favorable, the disease can cause up to 90% loss. Foliar symptoms (symptoms on leaves) may start with water-soaked lesions spanning the length of the leaf blade, which gradually become blighted resulting in desiccation and collapse of the tissue. Experiments have shown that the bacteria can move from leaves to the bulbs, thus protecting foliage is important to manage the disease.
The bacteria can overseason to infect onions in a number of different ways. Like many bacterial pathogens, P. ananatis can be seed-borne with infested seed serving as a survival mechanism as well as a means of dissemination. It has been demonstrated that P. ananatis can be both naturally seed-borne and seed-transmitted in onion. The significance of the bacterium's ability to colonize seed is uncertain, as most onion seed production sites are located in arid climates but extremely important to understand to manage the disease.
Although P. ananatis can be seedborne, the proposed primary mode of transmission is by two insect vectors. Two species of thrips, tobacco thrips (Frankliniella fusca (Hinds)) and onion thrips (Thrips tabaci), have the ability to transiently acquire and transmit P. ananatis and P. agglomerans . The bacterium can persist in a non-circulative manner in the gut of thrips for 128 h, allowing the vector to infect plants over an extended period of time.
P. ananatis can survive epiphytically and endophytically on a wide range of hosts. These alternative hosts can serve as a source of inoculum in fields where susceptible crops are grown. In Georgia alone, 25 weed species, including carpetweed (Mollugo verticillata), common ragweed (Ambrosia artemisiifolia), crabgrass (Digitaria sanguinalis), common cocklebur (Xanthium pensylvanicum), curly dock (Rumex crispus), Florida pusley (Richardia scabra), sicklepod (Cassia obtusifolia), stinkweed (Thlaspi arvense), Texas panicum (Panicum texanum), vaseygrass (Paspalum urvillei), wild radish (Brassica spp.), yellow nutsedge (Cyperus esculentus) and other multiple crop plants were found to harbor P. ananatis populations asymptomatically.
Pic Credit: Colton Tew
Onion cultivars resistant to Pantoea sp. are not commercially available. Use of certified onion seeds is encouraged to avoid introduction of Pantoea sp. inoculum in the production field. Planting early maturing or mid-maturing onion varieties are often recommended for growers. Late maturing varieties provide a larger window for infection and a potential epidemic to occur, which are favored by thrips pressure, hot and humid conditions, and lack of effective bactericides. Overhead irrigation should be avoided as it promotes bacterial spread compared with sub-surface or drip-irrigation. Controlling thrips population can be an effective management strategy to reduce center rot incidence as these vectors play an important role in bacterial transmission.
Center rot management in onion fields relies heavily on copper applications mixed with an ethylenebisdithiocarbamate fungicide (EBDC), such as mancozeb, which growers may apply weekly as a protectant. In addition, researchers found P. ananatis strains to be copper-tolerant indicating overuse and potential risk of insensitivity to this chemistry. Repeated applications of copper sprays during susceptible growth stages can be effective only to a limited extent and does not offer a robust solution to the problem. Perhaps the inefficacy of these sprays could be due to thrips preference to colonize certain parts of the onion plant, e.g. the basal meristems (neck region).
The implementation of successful weed management strategies are important in reducing P. ananatis inoculum in the field. By reducing weeds, growers can potentially reduce initial inoculum.
Hairy fleabane grows about 4 feet, branches from the bottom and leaves are pubescent also stems are covered with stiff hairs. The growth habits of horseweed (marestail) are different in that they grow up to 10 feet tall and branches from the upper half of the plant.
You can find Fleabane flowering right now in the Yuma area especially the Yuma Mesa. The IPM Team has received calls from our friends PCAs and growers stating that the Fleabane survives the application of Glyphosate, which has been reported for both conyzas. So, we got some trials in progress looking for options to control Fleabane as well as other weeds that have shown tolerance to Roundup such as White sweet clover or malva in different crops.
The results from the evaluations will be shared in this newsletter.
1. UC/IPM Retrieved from http://ipm.ucanr.edu/PMG/WEEDS/hairy_fleabane.html