Diamondback Moth in the Desert – Status and Update
Diamondback moth (DBM) continues to appear in traps and fields throughout the desert. Pheromone traps indicate that moth activity increased in the past week or so in Tacna/ Roll, Dome Valley, Yuma Valley and Bard/Winterhaven (See DBM Trap Network). One trap in Roll adjacent to a direct seeded broccoli field had > 15 moths / trap/ night over the past week, while a trap in Bard next to a newly-transplanted cauliflower field had almost 21 moths / trap / night. To me these are high numbers and suggest significant DBM population development. However, in most cases, DBM larvae are absent, or present in very low numbers in fields, and to date no PCAs have reported difficulty controlling DBM. On the Yuma Ag Center (YAC) , we have a very robust DBM population in our direct seeded broccoli. In fact, the DBM numbers are higher than cabbage looper and beet armyworm combined. I have never seen DBM this abundant during September /October. In the last update we pointed out that DBM could be found mining small seedling broccoli plants. Now that those plants are getting larger, we have noticed that small DBM larvae (2nd instar) can often be found feeding within the terminal leaves of the plants. It’s important to sample this part of the plant as one can easily miss DBM infestations if you fail to look closely within the plant terminal (see image below). The good news (at least for an extension entomologist) is that these high populations are allowing us to conduct field trials that we normally would not be able to this time of the year. Our results to date are showing some very surprising, but encouraging results. Tray drench and soil shank applications of Verimark and Coragen are showing > 35 days of control on cauliflower and broccoli. In our foliar spray trial, all of the spray treatments provided significantly better control of DBM larvae compared to the untreated check following 2 spray applications (see Efficacy of Foliar Insecticides Against Diamondback Moth - Fall 2017). This is interesting because the pyrethroids, Lannate and Coragen did not control DBM in similar spray trials conducted in spring 2017. The fact that the DBM in the present study were susceptible to these insecticides strongly suggests that the population presently found on the Yuma Ag Center is different than the population found on the farm in spring of 2017. The resistant population last spring originated from a local nursery, whereas we are not certain where the current susceptible population originated from. Note: these results are specific to the population at YAC, and may or may not reflect similar susceptibility for other DBM populations found in the desert. However, it will be important to measure insecticide susceptibility in as many populations in the Yuma area as possible. Thus, if you have DBM present in any of your fields, please contact us and we can collect larvae before you treat.
In response to the recent outbreaks of Diamondback moth (DBM) , Plutella xylostella in Yuma, we have established a pheromone trap network designed to monitor the activity and movement of adult populations of DBM. PCAs have had difficulty controlling DBM in cabbage, broccoli and cauliflower since October. Traps have been placed in Roll, Wellton, Dome Valley, Gila Valley and Yuma Valley in locations where cole crops are presently being grown or in areas where infestations were known to occur this fall.
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 captures have steadily decreased over the past 2 weeks, and areawide about average for late-October.
Beet armyworm:
Trap counts reached their highest levels so far this season, particularly in Tacna, Wellton and Yuma Valley, and about average for late October.
Cabbage looper:
Cabbage looper numbers decreased areawide, and are still below average for this time of the year.
Diamondback moth:
Sporadic DBM activity in low numbers throughout the area, trending well below average for late October.
Whitefly:
Adult movement increased in the past 2 weeks and above average for late October.
Thrips:
Thrips adult activity peaked in the last two weeks, and trending below average in October.
Aphids:
Winged adults continue to be captured for the season, consistent with heavy winds from W-NW. Aphid captures thus far have been well below average.
Leafminers:
Adult activity decreased in most areas, and trending about average for late October.