We continue to trap diamondback moth (DBM) in traps throughout the area. We currently have 57 pheromone traps stretched from Texas Hill to San Luis. In the past few weeks we’ve seen moth activity decrease in general in traps at Texas Hill, Tacna Roll, Wellton, Dome Valley and Bard/ Winterhaven. In contrast, we’ve recorded increases in trap captures in some traps in the Gila and Yuma Valleys. (See DBM Trap Network).
One trap located at the Yuma Ag Center captured more than 28 moths /trap per night last week. That trap is adjacent to several acres of untreated broccoli that are infested with DBM larvae. However, most traps throughout the Yuma area are not catching anywhere near those numbers of moths, and not surprising, the fields adjacent to these traps have little to no DBM larvae. Furthermore, to date no PCAs have reported failures in controlling DBM like they experienced last fall. On the Yuma Ag Center, we have had plenty of DBM in our cole crops. This allowed us to conduct a number of efficacy trials with both soil and foliar applied insecticides. In our Verimark tray-drench trial in cauliflower, we observed DBM control for almost 40 days. Soil shank applications of Verimark in broccoli provided DBM efficacy for ~ 30 days. Our foliar spray trials consistently showed that the all conventional insecticides we evaluated provided significantly better control of DBM larvae compared to the untreated check following multiple spray applications (for results see Efficacy of Foliar Insecticides Against Diamondback Moth, Fall 2017 andEfficacy of Foliar Insecticides Against DBM and Whitefly). On the other hand, we found it more difficult to control DBM with organically-approved biopesticides (see Organic Insecticide for DBM Control on Broccoli, Fall 2017).
An important lesson was learned from these studies. This fall DBM was not an issue, unlike last season. We’re obviously dealing with a different population this year. It is highly likely that the present DBM populations found at the Yuma Ag Center and throughout the Yuma area moved into the area via monsoon/tropical storms. I base this on the fact that the DBM have been easy to control with our standard insecticides (unlike last season), and we have been trapping moths adjacent to direct-seeded broccoli fields since early September. We’ve also been trapping them next to fallow fields. This clearly suggests that the DBM in Yuma this fall did not exclusively arrive on transplants, nor did last season’s resistant population over-summer here. I believe this explains why DBM is seldom a problem for Yuma PCAs. DBM populations have to migrate in and over the past 20 years or so, they have been susceptible to our industry standards. The exception would be last year, where the DBM outbreaks we experienced were associated with a resistant population that moved into fields from infested transplants. One last thing to consider: it is possible we may see additional DBM move into the area via winter storms from the coast. Hopefully we can pick up such movement with our traps.
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:
Corn earworm:
CEW moth counts remained low over the past 2 weeks across all locations and about average for this time of the season.
Beet armyworm:
Trap counts decreased in most locations, and well below average for late-November. Most activity in Yuma Valley.
Cabbage looper:
Cabbage looper trap counts remained low in most areas but increased in the Yuma Valley. Activity below average for late November.
Diamondback moth:
Adults peaked in Bard, Gila and Yuma Valleys and slightly above average for this time of year. Traps located adjacent to cauliflower seed crops had the highest trap captures
Whitefly:
Adults remains active in Dome Valley and Roll consistent with melon crops completing harvest, but below average movement for this time of season.
Thrips:
Thrips adult movement decreased in most locations last week, and most active in Dome Valley, Wellton, and Tacna. Activity about average for mid-November.
Aphids:
Aphid movement peaked so far this season with highest activity in Dome Valley, Bard, N. Yuma Valley and N. Gila Valleys over the past 2 weeks. Activity average for this time of year.
Leafminers:
Adult activity increased sharply in the Dome, Yuma, and Gila Valleys, about average for this time of season.
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
