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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.
This is the second in a series of articles discussing technological advances being made by manufacturers of automated thinning and weeding machines. This is a fast-moving space and innovations are entering the marketplace constantly. One of these on the near-term horizon is a robotic thinner/weeder being developed by Tensorfield Agriculture, San Francisco, CA that uses hot vegetable oil to kill weeds. The idea is to spray a “micro-dose” of hot oil (320 °F) onto the targeted plant/weed with high levels of precision. Why vegetable oil and not water or steam? Sufficient levels of heat from any source will rupture cell membranes and kill plants, but the advantage of vegetable oils is that they adhere to plant surfaces better than water and can be raised to much higher temperatures before boiling. Soybean oil for example, has a boiling point of 450 °F which is much higher than that of water at 212 °F. Consequently, vegetables oils transfer more heat faster than hot water and kill plants more effectively. A limitation of steam is the difficulty in concentrating the heat energy onto the target plant.
The concept of using hot vegetable oil to control weeds with an automated machine is not new and has some merit. Vegetable oil degrades naturally in the soil and thus can be used in commercial and organic crop production. Researchers at UC Davis developed and tested a prototype, hot oil based micro-dosing sprayer for automated weeding in tomato crops (Giles et al. 2005, Zhang et al., 2012). They found the technique effective at controlling weeds (>90%), but computing speeds were too slow for the integrated automated weeding machine to be commercially viable at the time.
Tensorfield Agriculture is rejuvenating the idea using modern computers, artificial intelligence and automation. The company has built a micro-dosing sprayer that delivers heated oil to target weeds at the 1/2” scale level of precision (Fig. 1). The sprayer assembly is mounted on an autonomous, robotic platform (Fig 2 - please note that the robotic platform depicted is a first-generation design developed for testing and debugging purposes and that a commercial style unit is forthcoming). Computer imaging and artificial intelligence are used to detect crop plants and weeds. The company will be testing and debugging the system this winter in California with carrot, spinach and romaine crops. The aim is to have prototype commercial systems available for the spring of 2021.
Some of you may have visited Tensorfield Agriculture’s booth or seen their technical breakout-session presentation at the 2020 Southwest Ag Summit in Yuma, AZ. They have an interest in working in the Yuma area and with the University of Arizona. It will be interesting to see how this technology progresses over the winter and how killing weeds with heated vegetable oil may benefit weed management systems.
As I mentioned, automated thinning and weeding technologies are advancing at a very rapid pace. If you know of a new technology that would be of interest and appropriate for this publication, 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.
Fig. 1. Custom built sprayer modules for delivering high temperature vegetable oil (350 °F) to kill weeds organically. The units are designed for precision weed control (1/2” scale of resolution) by Tensorfield Agriculture, San Francisco, CA (Photo credit – Tensorfield Agriculture).
Fig. 2. Autonomous robot for thinning and weeding using heated vegetable oil. The unit is a first-generation prototype designed for testing and debugging purposes by Tensorfield Agriculture, San Francisco, CA (Photo credit – Tensorfield Agriculture).
Giles, D.K., Lanini, W.T. & Slaughter, D.C. 2005. Precision weed control for organic and conventional specialty crops. In Buy California Crop Block Grant Program Final Report. Sacramento, Calif.: California Department of Food and Agriculture.
Zhang, Y., Staab, E.S., Slaughter, D.C., Giles, D.K. & Downey, D. Automated weed control in organic row crops using hyperspectral species identification and thermal micro-dosing. Crop Protection 41: 96-105.
Weeds are one of the most visible of all agricultural pests. They can’t move or hide and once established often stick up over the crop. Just one weed in a 10 acre field is annoying to look at. With insects and diseases, the damage is often more visible than the pest. That is not the case with weeds. A moderate weed infestation is approximately 10 weeds per square foot. If a herbicide produces 90% control, that leaves 1 weed per square foot or 43 weeds per acre. Without an untreated check, this can look like the herbicide failed! It is easy to leave an untreated spot in a field and it is well worth doing. Many applicators do so unintentionally because of skips, powerlines and other causes. They help determine crop injury and weed control. Here are some examples of what various levels of control looked like from one of our cole crop trials: