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.
We are on the final section of virus transmission. Virus transmission by insects is one of the most efficient and economically important transmission in agriculture. When you have insects in your crops, not only you are losing your crops because of feeding/chewing by insects, a lot of insects also act as a vector of plant viruses.
Seven out of 29 orders of insect feeding on living green land plants are vectors of plant viruses.
Insect transmit viruses in 4 distinct modes:
Non persistent transmission: The insects can acquire the virus in a matter if seconds/minutes and they are immediately viruliferous. The virus in retained in the stylet of the insect and are transmitted to the next plant the insect feeds on. The virus is retained in the vector only for few minutes and is lost after insect molting. Most viruses transmitted by aphids are non persistent. So when you see few aphids in your melon field and see cucumber mosaic virus symptoms 1-2 weeks later in your field, don’t be surprised. Aphids are efficient vectors, and since viruses are systemic it takes anywhere from few days to 2-3 weeks for the plants to show symptoms. Thus it is very important to manage insects in the field even if you don’t think the ‘pressure’ is not as high.
Semi-persistent transmission: The insects can acquire the virus in minutes/hours and there is no latent (incubation) period in the insect. The virus can stay in the insects foregut for hours and is lost after insect molting. Some species of aphids and whiteflies fall in this category. Example: Cucurbit yellow stunting disorder virus in melons transmitted by whiteflies.
Persistent circulative: Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus for weeks. Virus can be present in the vectors hemolymph but there is no multiplication of virus in the insect body. Vectors in this transmission includes: Aphids, leafhopper, whiteflies, treehopper.
Example: Beet curly top virus transmission by beet leafhopper
Persistent propagative: Insects have to feed on virus infected plants for hours/days to acquire the virus and the virus has to incubate for hours/days in the insect. After insect can transmit the virus throughout its lifespan. The virus can multiply in the vector system and often times the virus particles are also passed on to the insect offspring. Tomato spotted wilt virus is transmitted on persistent propagative manner by 9 different species on thrips.
Save the Date : 2024 Plant Pathology Workshop
When: August 29th 8AM-12 PM ( breakfast and Lunch provided by Gowan Company and BASF)
Where: Yuma Ag Center, 6425 W 8th Street
What will covered: Plant Pathology program Updates, past season field trial results (we
have some exciting results to share), Q&A to help better Plant pathology program,
Industry panel discussion for all your industry related questions! See you in few weeks!
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
