As is typical this time of the year, the weather has finally broken and it feels like “winter” has arrived in the desert. With cooler temperatures, you can expect a steady decline in insect abundance until temperatures begin to warm up again. Last fall, I felt that the insect pressure on produce and melons crops in the Yuma was as heavy as I’d seen in many years. This year it didn’t seem quite as heavy, but I’ve heard many PCAs complain of heavy insect pressure, particularly during November which was unusually warm. A quick look at both recent and historic data on pest abundance recorded from our research plots here in the Yuma area suggests that insect pressure this fall was a little lighter than last year, but still heavier as compared to previous years. First, whitefly adult numbers on fall melons and produce were extremely high in August, but monsoon rainfall in early September appeared to suppress their numbers for a couple of weeks thereafter. Much like last fall however, untreated melons plots on the Ag Center wilted and died rapidly as a direct result of heavy whitefly infestations. Sticky trap captures of whiteflies near cantaloupe fields from Wellton to Texas Hill, were lower than last fall, but were still higher than the previous four years. However, numbers of adults caught on traps in the Roll/Tacna area were higher in late-August this fall than we’ve ever recorded. Similarly, CYSDV incidence at harvest in cantaloupe fields in these areas was very high this fall. In all cases, high traps catches were found in melon fields in near proximity to cotton fields. Based on counts from untreated lettuce at the Yuma Ag Center, beet armyworm and cabbage looper population abundance this fall was slightly lower than last fall. However, worm pressure the past two years was higher than we’ve seen the previous 5 years. Populations began infesting plots in early September, and remained steady throughout October and into early November. Egg deposition and larval development has only recently declined significantly. Corn earworm numbers were lower than what we observed last year, but could still easily be found in untreated plots. Finally, Bagrada bug infestations were abundant in untreated broccoli plots at YAC for the 4th consecutive year. As expected, low-moderate population appeared in early September, but increased steadily through September and peaking in early October. Overall, the peak numbers observed were not as high as either 2010 or 2012, but much higher than in 2011. The infestation levels in our untreated broccoli plots this year remained at damaging levels throughout October and into mid-November. This is the first year we have seen Bagrada bugs this abundant in November. In contrast, similar to last fall, western flower thrips population numbers have been relatively low, but unlike last fall, we’ve been picking up winged green peach aphids and colonies on sticky traps and in untreated lettuce plots since early November. How these aphid and thrips numbers will translate into potential population pressure in January and February is unknown, but you should anticipate their abundance as usual. I’m often asked why we see such differences in insect pest numbers each year. It’s obviously a very complex question, and I don’t have a good answer. The bottom line; insect abundance and outbreaks are dictated by many abiotic, biotic and crop management factors within our cropping system. Moreover, it’s nearly impossible to consider all the factors necessary to draw a reliable conclusion. Nonetheless, graphics showing these recent trends in Whitefly, CYSDV, Lep Larvae and Bagrada abundance can be found at Pest Abundance on Desert Produce and Melon Cops in 2013.
Widely accepted definition of a living organism “A living organism has a cellular structure and is manifest by growth through metabolism, reproduction, and the power of adaptation to the environment through changes that originate internally”. Viruses are not cellular and do not metabolise, but they reproduce and adapt.
A virus is a set of one or more nucleic acid template molecules, normally incased in a protective coats of protein or lipoprotein and is able to organize its own replication but only within a suitable host cells. Record of plant viruses do not go as far as human viruses, but plant viruses have caused considerable loss in agriculture system.
One of the most common virus we see in agriculture system in todays world is Cucumber mosaic virus(CMV). CMV belongs to family Bromoviridae. The genome size of cucumber mosaic virus (see pic) is about 8000 to 9000 nucletotide bases (1 base=1 letter of AGTC). The genome size of Covid19 Coronivirus is about 30,000 bases and the genome size of human DNA is 6.4 billion bases.
CMV has a very wide host range and is transmitted by aphids in nonpersistent manner (stylet borne). This means that the aphids acquire the virus particle in their stylet within seconds of feeding in infected plants, hop on to next plant and start feeding on next plant. The virus is transmitted to the next plant immediately.
Next is incubation period. Viruses cause systemic infection. It can take anywhere from few days to few weeks from initial entry of the virus to symptom exhibition in your plants. The severity of symptoms varies depending on many factors. The age of plant (infection stage), the general plant vigor (health), varietal susceptibility, conducive environment (viruses express better in colder weather than hot weather), a plant that has already been infected with other viruses (preesisting condition) are to name a few.
Attachment – the virus attaches itself to the outside of a new plant cell
Penetration – the protein pushes the nucleic acid strand into the plant cell
Replication – the viruses’ nucleic acid uses the plant cell DNA to make many new nucleic acid strands and protein sheathes
Assembly – the nucleic acid and protein assembly into millions of new virus copies
Release – the viruses leave the cell – at this stage the cell is normally dead and bursts releasing the viruses
Transmission – the viruses move using a vector to new cells to infect.
When you see the symptoms in your plants, the first thing you have to understand is virus infection is systemic. The best you can do to manage the virus is to limit the transmission (flatten the curve). Some viruses need a vector for transmission like insects and nematodes. Some viruses are mechanically transmitted from one infected plant to another. Washing field tools between plants/field whenever possible limits the transmission of virus. Soap, bleach, and disinfectants reduce transmission by protein denaturalization of the virus.
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: