May 4, 2022Spider Mites on Spring Melons 2022To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
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.
It’s October already and soon crews will be thinning and weeding vegetable crops in the Yuma area. Automated machines for performing these tasks are commercially available and being used more and more. Due to significant research and development efforts being made in this area, the technology used for these machines is advancing rapidly. In the next series of articles, I will be discussing the developments made over the summer by automated machine manufactures and what growers in the Yuma area can expect for the upcoming season.
The first technology I’d like to discuss is the autonomous weeding robot “Dino” manufactured by Naïo Technologies (Fig. 1). Briefly, the robot is an 84” wide, self-piloted, mobile power unit equipped with camera guided cultivating tools.
Last season, Naïo demoed the robot for growers in the Yuma area. It was the first year the French based company had operations in the U.S. One goal of the demonstrations was to evaluate the machine in U.S. conditions and get feedback from growers about how the machine might best fit Arizona production systems. The company learned that Arizona vegetable production is very intense and fast paced. Machine productivity and durability were important issues for viability and domestic acceptance.
The company took heed of these observations. Over the summer, the battery that powers the fully electric robot was upgraded to provide 10 hours of operation before needing to be recharged. This improved work capacity from 8 acres/day to about 10 acres/day (work rate = ~ 1 acre/hour). To further address the capacity issue, the company imported three additional machines so that multiple machines could be operated simultaneously in the same field. The new machines are more robust, being equipped with higher torque drive wheel motors and a sturdier frame. The company is also in the process of switching to an improved camera guidance system that should enhance crop tracking ability and close cultivation performance. Some may be aware that the company has developed an award winning active in-row weeding tool that cycles cultivating blades in and out of the crop row (Link). The device is undergoing further testing and is planned for commercial release in mid-2021.
Machines will be available this fall in the Yuma area for demos or for hire as a weeding service. It will be interesting to see how these advanced robots perform and benefit weed management. If you would like assistance evaluating these machines or conducting performance trials, please feel free to contact me.
 Reference to a product or company is for specific information only and does not endorse or recommend that product or company to the exclusion of others that may be suitable.
Heavy and widespread infestations of common purslane come up during ground preparation for lettuce every year. This occurs in fields that were kept weed free the previous year and is difficult to understand.
There are probably several reasons for this.
Common Purslane is very prolific. It has been reported that one plant can produce up to 240,000 seeds. The stems are so succulent that plants can remain viable and make seed even after it is uprooted.
Once seed is mature it can be viable for as long as 40 years. It has very small, hard seed that can remain dormant in the soil for ss long as 40 years .So you may have to control weeds that got into the field a generation ago..
Multiple perennial germinations
Common Purslane is supposed to be a summer annual, but it germinates multiple times all year in the low desert. It takes 12 hours after receiving moisture in the summer and 7 days in the winter, but it keeps germinating. It has to be controlled when it is less than 2” in diameter. If you wait until most of it germinates the early plants will be too big. If you spray or cultivate when all the emerged plants are small you will miss many that have yet to emerge. It is best to treat early and control the later emerging plants with a selective herbicide.
When common purslane is broken in pieces it can reroot at the nodes. Late cultivation often spreads this weed. Cultivation is not a good option when purslane is larger than 2”. Herbicides are a better option on big plants.
Purslane has a very small light seed. It moves in irrigation water and blows in the wind. Even completely clean fields are likely is be reinfested by seeds that are carried by water and wind into the field.
Considering the above factors, the best option for controlling common purslane may be preirrigation to germinate the weeds and early herbicide application or cultivation . Kerb and Prefar are both good on purslane. Prefar should be used at planting to incorporate it with a lot of water and Kerb should be used later to avoid leaching but don’t wait too long and risk germination of the weeds. Purslane germinates from shallow depths and split applications of Kerb may be a good option.