To my surprise, the diamondback moth (DBM) has already returned to the desert, and its timely occurrence happens to coincide with the establishment of early brassica transplants. I was not anticipating this early of an arrival; but they are definitely here. We began to pick up a few DBM moths in traps during the week of Aug 19-26. The last moths caught prior to this was in early July near a brassica seed crop. Then during the week of Aug 26-Sep 2, traps captured a higher number of diamondback moth adults in several areas. A total of 22 DBM adults were caught in 7 traps during that week. May not sound like a lot, but more than what I expected. In all but one location, the moths were caught in traps located adjacent to newly transplanted cauliflower or cabbage fields (See DBM Trap Network). Moreover, since Monday we have seen a sharp increase in moths captured, particularly in Dome Valley and Wellton. At one trap location in Wellton, I counted 31 moths captured over 2 nights. There have also been a number of reports from PCAs in the past 2 days of adult DBM flying within fields. The interesting thing is that no one has reported any eggs, larvae or feeding damage on transplants in the fields where these adults are being found. Could be that the Verimark tray drenches are preventing DBM colonizarition so far. Time will tell. This early moth activity seems unusual to me, but maybe it’s because I’ve been looking so hard for them. The key question is where did these adults come from? The answer is important as it may indicate whether we are dealing with the same diamide resistant population we battled in 2016-17, or a completely different population with resistance to some other chemistry or nothing at all. In my view, there are 3 potential points of origin for these DBM adults. 1) Local Residents - I’ve always assumed DBM would not be capable of spending the summer (mid-June to mid-August) in the desert because of the lack of a suitable host. Our trapping data appears to support this hypothesis since we caught no moths during this period. But don’t know for sure. 2) Hitchhikers – another potential source could be the transplants themselves. Very possible, and can’t be ruled out, but the transplants where moths have been reported/captured have originated from six different nurseries so far (4 from coastal CA and 2 local). Have not picked up any DBM adults in direct seeded broccoli yet, but time may tell. 3) Immigrants - We know that DBM are capable of migrating long distances in winds, and given the widespread occurrences of the moths so far, it may be possible that recent storms may be bringing some of them into the area from the south. It may just be a coincidence that the large increase in moth activity in the last 2-3 days follows a tropical storm disturbance that moved through the area this pest weekend? We may never know the origin, but trust me we will continue to investigate. The bottom line: PCAs and growers should anticipate an early occurrence of DBM this season and prepare accordingly. For more information of managing DBM on fall crops see Guidelines for Diamondback Moth Management in Fall Cole Crops.
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: