It’s that time of the year when I commonly receive calls concerning spider mites on melons. Twospotted spider mites, Tetranychus spp, are widespread on melon crops throughout the southwestern U.S, and in my experience, outbreaks are more common on watermelons than other netted and mixed melon varieties. However, problems can certainly occur on all melon types. Under ideal condition, spider mite populations can increase dramatically. The hot dry weather we are now experiencing can be favorable for mite infestations. For example, mites can go from egg to adult in about 37 days when temperatures average about 60° F, but at average temperature of 86° F, it only requires 7 or 8 days for them to complete their lifecycle. Several natural enemies (including predatory mites and thrips, minute pirate bugs and lacewings) play an important role in regulating mite populations below economically damaging levels. Predators can maintain mite populations in check by feeding on slowly developing mite populations. However, when temperatures become hot and dry, the mites can quickly out run the predator population due to their rapid reproduction. Good irrigation and fertilization management can enhance plant tolerance to mites. Although economic thresholds have not been adequately established for spider mites in the low desert, treatment with an acaricide is recommended when significant webbing can be found on leaves and predatory mites and thrips are absent. A number of effective miticides are available for melons including Agri-Mek, Oberon, Acramite, Miteus, Zeal and sulfur. If leafminers are present in the field, Agri-Mek is a good option. If whiteflies nymphs are beginning to build on older leaves, Oberon or Minecto Pro (abamectin+cyantraniliprole) may be an effective option. For more information on mites and their management please visit: Spider Mites on Melons.
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:
Results of pheromone and sticky trap catches can be viewed here
Corn earworm: Moth activity remains seasonably low, but noticeably increased again in
the North Yuma Valley. Below average compared with previous seasons.
Beet armyworm: Moth activity increasing comparable to fall numbers, particularly in Tacna, Dome Valley and Gila Valley; consistent with reports of larvae in fields. Slightly about average compared with previous seasons.
Cabbage looper: Similarly, cabbage looper moth activity increasing Tacna, Dome Valley
and Gila Valley and remains above average for this time of the year. Above average compared with previous seasons.
Whitefly: Adult movement is relatively absent; typical for this time of the season.
Thrips:Thrips activity remains seasonably low; but is increasing in the Yuma
Valley. Well below average compared with previous seasons
Aphids: Area-wide aphid flights decreased in the last 2 weeks. Most aphids found on traps were green peach aphid. Below average movement compared with previous seasons.
Leafminers: Adult activity remains seasonably low in all locations, and below average for this time of year.
Diamondback moth:
Diamondback moth (DBM) counts in pheromone traps trended downward over the past 2 weeks, and remained low in many locations with the exception of the N. Yuma Valley. Across all locations, trap counts remain higher now than trap counts recorded this time last year, but significantly lower than 2017 following the fall 2016 outbreak.