With the changing weather patterns (wind actively blowing out of the north and west), PCAs can anticipate an increase in winged (alate) aphids showing up on desert produce crops. Based on my observations over the past two decades, this is an annual occurrence with our key aphid pests on produce. Aphids that typically infest leafy vegetables do not over-summer in the desert. Rather, winged adults migrate into our cropping system from mountainous regions of California via wind currents during November/December. Once the aphids reach our desert valleys, they typically move from crop to crop until they find a suitable host to feed and colonize on. But don’t panic just because you suddenly find a few winged aphids on the plant. It is not uncommon to find winged aphids on lettuce or broccoli that do not colonize on the crop. An example of these would be cabbage aphid which will colonize and infest cole crops but not lettuce, spinach or celery. Other examples would include aphids that colonize small grains (i.e., corn leaf aphid) or alfalfa (i.e., blue alfalfa aphid). Because these aphid species will not colonize produce crops, it is important to be able to distinguish them from the aphids that do colonize and require management to prevent problems at harvest (i.e., green peach aphid, foxglove aphid, lettuce aphid, cabbage aphid). Proper aphid ID can also influence your choice of insecticide, but more on that in a later update. As mentioned in the last update, PCAs have been finding small colonies of cowpea aphids showing up on frame leaves in lettuce. That is a common occurrence every fall. Not to worry, experience has shown us that although small cowpea aphid colonies may be found on lettuce, the populations generally stay low on the plant on the frame leaves and rarely increase to levels causing contamination issues. But you never know. So keep an eye for these guys, as our El Nino weather this year may be more conducive to their development than normal. Local research has shown that aphids tend to be more abundant at harvest in lettuce in years with higher winter rainfall than average (see Impact of Lettuce Planting Date on Aphid Contamination). Also, don’t forget that proper aphid identification is important; it can save a PCA time and money, and prevent unnecessary insecticide applications. If you find an unusual aphid in your produce, don’t hesitate to drop it by the Ag Center and we’ll get it identified for you. But if you want to be fast and accurate you might use the attached publication Aphid Identification in Desert Produce Crops that may assist you in identifying winged and wingless (apterous) aphids important in leafy vegetables and cole crops.
Plant viruses cannot penetrate the intact plant cuticle and cellulose cell wall that acts as barrier to infection. The virus overcomes the problem by either avoiding the need to penetrate (example seed transmission) or by using the wound in plants as infection site, or transmission by insects, nematodes or fungi as a vector.
Mechanical transmission involves the introduction of infective virus or viral RNA into the wounds of plants. Viruses such as Tobacco mosaic virus (TMV), Potato virus X are highly stable, and reach high concentration in plants. As you all know TMV can readily contaminate hands, clothings, and implements and can be spread by worker. TMV can even spread mechanically by tobacco smokers as the virus is present in cured tobacco leaves.
Mechanical transmission is of great importance. In field and greenhouse, great amount of caution has to be implemented to not transmit the infection. Field sanitation, tool sanitation is very important to avoid the spread of virus.
However, in experimental world mechanical transmission is a very useful tool to study viruses. Mechanical inoculation of virus to a heathy host plant is done for assays, to produce local lesions, in the propagation to of viruses for purification, in host range study, diagnosis, and to understand the interaction between virus and susceptible cells.
Seed transmission: About 1/7 th of the known plant viruses are transmitted through seeds. Different viruses have different host ranges (the plants that they can infect). Tobacco mosaic virus, Cucumber mosaic virus are some viruses with a very wide host range and they may not be seed transmissible in all plants they infect. Seed transmission plays a huge role in virus epidemiology. Not only they can be a primary source of infection, leading to an epidemic in the field upon conducible environment, seed transmission is an effective way for long distance travel of the virus, thus introducing the virus to new places. You have heard of USDA regulations/restrictions on different crops, from certain foreign countries to avoid introduction of infected seeds/plant materials.
Seed transmission can occur simply by contamination of seeds, as in tomato seeds by Tobacco mosaic virus. This can be readily inactivated by seed treatments.
The second type of seed transmission occurs when the virus is present in the embryo tissue that can happen prior to fertilization or takes place at pollination. Pea seed-borne mosaic virus is a well studied plant virus in this category.
Pollen Transmisison: Some viruses are transmitted from plant to plant via pollen. As in seed transmission, pollen transmission has two mechanisms, gametic infection of embryo and direct infection of mother plant.
Vegetative propagation: An important horticultural practice, and unfortunately a very effective method for perpetuating and spreading viruses. In clonally propagated plants, an infected mother plant which could be asymptomatic could be used to make hundreds and thousands of daughter plants, which will all have the virus. Any vegetative parts such as bulbs, corms, runners, and cutting will be infected.
Grafting: Essentially a form of vegetative propagation, once the organic union has been established and plants (Scion and Stock) function as a single plant. In experimental front, grafting is used as a virus transmission methods, when all other methods fail.
Controlling Fusarium Wilt of Lettuce Using Steam Heat – Trial Initiated
Earlier this week, we initiated a trial examining the use of band steam for controlling Fusarium wilt of lettuce. The premise behind this research is to use steam heat to raise soil temperatures to levels sufficient to kill soilborne pathogens. For Fusarium oxysporum f. sp. lactucae, the pathogen which causes Fusarium wilt of lettuce, the required temperature for control is generally taken to be > 140°F for 20 minutes. Soil solarization, where clear plastic is placed over the crop bed during the summer, exploits this concept. The technique raises soil surface temperatures to 150-155˚F, effectively killing the pathogen and reducing disease incidence by 45-98% (Matheron and Porchas, 2010).
In our trials, we are using steam heat to raise soil temperatures. Steam is delivered by a 35 BHP steam generator mounted on a custom designed elongated bed shaper (Fig. 1). Preliminary results were encouraging. The device was able to increase the temperature of the top 3” of soil to over 180°F at a travel speed of 0.5 mph as shown in this video of the machine in action (shown below). These temperatures exceed that of those known to control pathogens responsible for causing Fusarium wilt of lettuce (> 140°F for 20 minutes).
Stay tuned for final trial results and reports on the efficacy of using steam heat to control Fusarium wilt of lettuce.
If you are interested in evaluating the technique on your farm, please contact me. We are seeking additional sites with a known history of Fusarium wilt of lettuce disease incidence to test the efficacy and performance of the device.
References
Matheron, M. E., & Porchas, M. 2010. Evaluation of soil solarization and flooding as management tools for Fusarium wilt of lettuce. Plant Dis. 94:1323-1328.
Acknowledgements
This project is sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support.
A special thank you is extended to Cory Mellon and Mellon Farms for allowing us to conduct this research on their farm.
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:
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
