Now that the days are getting longer and the temperatures a bit warmer, aphids are likely to be more common. Experience has also shown us that aphid population growth begins to increase significantly once the weather warms in January. Our local weather forecast suggests that temperatures will be in the upper 60’s with moderate nighttime lows in the mid-40’s for the next two weeks. Assuming the weather service is correct, these conditions may be ideal for aphid population growth. Similarly, with the rainfall we experienced in December, I would keep an eye for aphid pressure to start picking up on your lettuce, celery, spinach and cole crops. Previous research has shown us that aphids tend to be more abundant during mild, wet winters.
So far, aphid appear to be ahead of schedule. Aphid flights peaked in mid-December which is a little later than what is expected (see graph below). There have been several reports of aphids showing up in both conventional and organic produce in the past 2 weeks, and at the Yuma Ag center, we have found enough green peach aphids colonizing small lettuce to initiate efficacy trials. Consequently, PCAs should be extra vigilant in scouting for aphid colonies. Be on the lookout for foxglove aphid too; they tend to be more biologically active under warmer temperatures (see Aphid ID Guide). If a field has been treated with imidacloprid at planting, then chances are you are in pretty good shape on young produce. However, imidacloprid doesn’t last all season. Regardless of whether you find green peach or foxglove aphid, the key to effective aphid management with foliar insecticides is to initiate sprays at the time apterous (wingless) aphids begin to colonize. The action threshold is the presence of aphid colonies on 5-10% of plants sampled. Of course, this requires diligent scouting and sampling. On older lettuce, make sure you thoroughly examine developing heads/hearts. Fortunately, PCAs have many alternatives for aphid control (see Aphid Management on Desert Produce_2020). So, keep your eyes open for new aphid colonies and treat early to keep ahead of the populations. For more information on aphid management see: Keys to Aphid Management in Leafy Vegetables.
DISEASE: Center Rot of Onion
PATHOGEN: Pantoea ananatis, Pantoea agglomerans, Pantoea alli and Pantoea stewartii subsp. indologenes
HOSTS: Onion (Allium cepa L.), garlic (Allium sativum L.), shallots (Allium cepa var. aggregatum L.), leeks (Allium ampeloprasum L.), chives (Allium schoenoprasum L.).
Symptoms and signs
Center rot of onion has not been a major problem in the desert southwest but when the environment is favorable, the disease can cause up to 90% loss. Foliar symptoms (symptoms on leaves) may start with water-soaked lesions spanning the length of the leaf blade, which gradually become blighted resulting in desiccation and collapse of the tissue. Experiments have shown that the bacteria can move from leaves to the bulbs, thus protecting foliage is important to manage the disease.
The bacteria can overseason to infect onions in a number of different ways. Like many bacterial pathogens, P. ananatis can be seed-borne with infested seed serving as a survival mechanism as well as a means of dissemination. It has been demonstrated that P. ananatis can be both naturally seed-borne and seed-transmitted in onion. The significance of the bacterium's ability to colonize seed is uncertain, as most onion seed production sites are located in arid climates but extremely important to understand to manage the disease.
Although P. ananatis can be seedborne, the proposed primary mode of transmission is by two insect vectors. Two species of thrips, tobacco thrips (Frankliniella fusca (Hinds)) and onion thrips (Thrips tabaci), have the ability to transiently acquire and transmit P. ananatis and P. agglomerans . The bacterium can persist in a non-circulative manner in the gut of thrips for 128 h, allowing the vector to infect plants over an extended period of time.
P. ananatis can survive epiphytically and endophytically on a wide range of hosts. These alternative hosts can serve as a source of inoculum in fields where susceptible crops are grown. In Georgia alone, 25 weed species, including carpetweed (Mollugo verticillata), common ragweed (Ambrosia artemisiifolia), crabgrass (Digitaria sanguinalis), common cocklebur (Xanthium pensylvanicum), curly dock (Rumex crispus), Florida pusley (Richardia scabra), sicklepod (Cassia obtusifolia), stinkweed (Thlaspi arvense), Texas panicum (Panicum texanum), vaseygrass (Paspalum urvillei), wild radish (Brassica spp.), yellow nutsedge (Cyperus esculentus) and other multiple crop plants were found to harbor P. ananatis populations asymptomatically.
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:
A new addition to the Veg IPM Updates this season is a pheromone/sticky trap monitoring network spanning the major growing areas in Yuma County. The project is designed to measure the activity and movement of adult populations of a number of key pests. The project is being funded by the Arizona Iceberg Lettuce Research Council, and will hopefully provide an indication of when pest activity (e.g., corn earworm moth flights) is increasing based on pheromone / sticky trap captures. The data is not intended to indicate field infestations, as trap data is largely a reflection of adult movement. If nothing else, the data may make PCAs aware of increased pest activity in some areas and encourage intensified scouting in susceptible produce fields. The pests being monitored include: corn earworm, tobacco budworm, beet armyworm, cabbage looper using pheromone traps; aphids, thrips and whiteflies using yellow sticky traps. A total of 8 trapping locations have been established in the following areas (approximate location):
Roll – 38E and Co. 5th St.
Wellton - 28E and Co. 10th St.
Dome – 18E and Co. 6th St.
S. Gila – Hwy 95 and Ave 10 E
N. Gila –Laguna Dam Rd and Co. 3rd St.
YAC – Somerton Ave and Co. 8th St.
YV mid – Ave C1/2 and Co. 15th St.
YV south – Ave H and Co. 19th St.
Traps will be checked weekly and data will be made available in the bi-weekly Veg IPM updates. If a PCA or grower is interested in weekly counts, those can be made available by contacting us. Below are the results of our first week of pheromone trapping which began on Sep 7th. Also included are whitefly and thrips counts from a few local areas initiated thus far.