Now that the produce season is winding down and melon production is well under way, PCAs should be thinking about leafminers on their spring cantaloupes, honeydews and watermelons. As temperatures continue to increase, the threat from leafminers also increases. Our area-wide monitoring network for most of the season has shown that leafminer numbers have been very light, however trap catches this past week showed last that leafminer adults were quite active based on trap counts. Interesting in many trap locations, both Liromyza sativae and L. trifolii were found on traps. This is significant because L. trifolii is typically more difficult to control than L. sativae. Leafminers can cause significant economic damage to melon plants, particularly on late spring and summer crops. Mining of leaves by the larvae can cause direct injury to seedling plants by removing chlorophyl and reducing the plants photosynthetic capacity. Mines and feeding punctures can also produce an entrance for pathogenic organisms. Excessive leaf mining in older plants can cause leaves to dry and defoliate, resulting in sun burning of fruit and reduction in yield and quality. In severe infestations, leafmining may cause plant death, particularly to seedlings or transplant watermelons. Damage to mature plants can occur when attempting to hold the crop longer for extended harvests. The good news is that a number of insecticide products are available that can effectively control both leafminer species. The most effective products are those that work via tranlaminar activity and can penetrate the leaf surface where they contact or are ingested by the developing larvae. These include Radiant (5-7 oz/ac), Coragen (5-7 oz/ac), Voliam Xpress (9 oz/ac), Exirel (15-20 oz/ac), and Agri-Mek SC (3.0 oz) that can effectively kill newly emerged larvae in the leaf mines before they cause significant damage. Because these products are selective, they have minimal impact on natural enemies that can help suppress leafminer populations. It is recommended that a penetrating adjuvant be added to these products to enhance translaminar movement of the product and larval control. For more information on leafminer biology and management please go to Leafminer Management on Desert Vegetables.
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 continue to be active, about average for this time of the season; particularly active in Wellton and Yuma Valley.
Beet armyworm:
Trap counts increased slightly in most locations, but well below average for early October. Most activity in Dome and Yuma Valley.
Cabbage looper:
Cabbage looper trap counts remain steady in most areas. Activity about normal compared to previous years.
Whitefly:
Adult movement increasing Dome Valley and Wellton, but down in many other locations; movement is below average for this time of year, but should anticipate increase movement with melon harvest beginning.
Thrips:
Thrips adult movement increased slightly in some locations, particularly in Wellton and Tacna, but remains below average for mid-September.
Aphids:
Aphid movement has been absent since early June typical with high summer temperatures, and the monsoon flow. Picked a up a few aphids last week with the break in weather and change in wind direction.
Leafminers:
Adult activity remains low in most location; below average for this time of season.
Diamondback moth:
Significant increase in moth activity in the last week, particularly in Dome Valley and N. Gila Valley in traps located adjacent to cauliflower and cabbage transplants.
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
