Our annual Lettuce Crop Losses Workshop was recently held in April and the results of the surveys continue to show consistent trends in insecticide usage on desert head lettuce. In general, the most commonly used insecticides in fall and spring lettuce correspond directly to the key pests that typically occur during these growing periods. By far, the pyrethroids, applied both as foliar sprays and chemigations, were the most commonly used insecticide class. No surprise there. Over the past 11 years, pyrethroid usage has remained steady. The reason for this is quite clear to me: pyrethroids are one of the most inexpensive and safe broad spectrum insecticides still available for use in tank-mixtures for effective control of flea beetles, crickets, plant bugs and some Lep larvae (looper and earworm). The overall use of OPs and carbamates continues to decline, but Lannate (methomyl) and acephate are still relied upon for thrips management. Their usage is being replaced primarily by several reduced-risk chemistries, of which the spinosyns remain the second most commonly used class of insecticides. In 2014-2015, nearly 95% of the lettuce acreage in our area was on average treated with > 2 applications of Radiant or Success. Their use against both lepidopterous larvae and thrips has remained steady since they were first registered. Foliar uses of Diamides (Coragen, Voliam Xpress, Vetica, Belt) were the third most commonly used chemistry in lettuce in 2014-2015. Since they were first registered in 2008, PCAs have steadily incorporated this new chemical class into their management programs. The use of Belt increased significantly this season, whereas soil uses of Coragen continue to decline. Ketoenol usage (Movento) on fall and spring lettuce increased this season likely due to heavier whitefly and aphid pressure. Another important class of chemistry used in fall and spring lettuce is the neonicotinoids driven primarily by soil-applied imidacloprid for whiteflies and aphids. The usage of imidacloprid on both fall and spring lettuce has increased markedly since 2009 and is used on almost 90% of the acreage, albeit at top of the label rates. Foliar neonicotinoid usage also increased last season, presumably due to heavier whitefly/aphid infestations in 2014-15. Finally, for the fifth season in a row, PCAs treated a greater percentage of their acreage with selective, reduced-risk products than with the broadly toxic, older chemistries. To view a summary of the estimated insecticide usage by chemical class, as well as the 12 most commonly used insecticides on head lettuce this past growing season, go to Insecticide Usage Summary in Arizona Lettuce 2015.

As celery starts to develop dense foliage, and with some favorable weather, it is that time of the year to watch out for late blight in celery.

Late blight of celery is caused by fungi Septoria spp. The disease is named late blight as it is mostly seen at the later in the growing season, but don’t be surprised if you see the symptoms in early season when the weather is conducive. Leaf spots are dark, circular to irregular in shape, and 3-10 mm in diameter. Dark colored fruiting bodies (pycnidia) ofthe fungus which form in the center of leaf spots give the spots a grainy appearance. In case of severe infection, large number of spots are formed and can significantly reduce yield. Sometimes, angular spots are seen as the symptoms are restricted by leaf venation. The stalk or petiole of the plants can also be infected and large number of pycnidia observed in the stalk. Pycnidia is basically huge amounts of asexual spores in dark fruiting bodies and are formed on the older lesions and their development is encouraged by moist weather.

The pathogen is seed borne but will survive in soil in decomposing celery tissue for months. Cool and wet weathers favor the disease. Temperatures below 75 F are conducive to disease formation. High humidity allows abundant production of spores and epidemics are initiated by splashing spores or by movement of spores by contact. Rain, heavy dew or fog, and sprinkler irrigation when temperatures are above 70°F encourage disease development; splashing water disperses spores and aids in spore germination and infection

Acquiring clean seeds is the best management practice for the disease. Hot water treatments are effective but might interfere the germination percentage. Clean cultivation, not planting new crop next to the infected crop field, crop rotation, and fungicides can be used to manage the disease.  Avoid sprinkle irrigation after symptoms are observed. Copper sprays can be used in organic farming.

We are conducting a celery trial this year to narrow down a susceptible variety, so we can conduct efficacy trial next season, so stay tuned!

 

This month in clinic:

The Yuma Plant Health Clinic will be closed from December 23rd to January 2nd. However, pictures of sick plants and text to my phone (928-920-1110) is a fair game!

The Plant Pathology team wishes you all Happy Holidays! 

Steam sterilization of soils is commonly used in plant nurseries and greenhouses for effective control of soilborne pathogens and weed seeds. The technique, however, is highly energy intensive as the entire soil profile is heated. This is too costly and slow to be practical for field scale vegetable production.  To reduce energy consumption and cost, use of band-steaming, where steam is applied only in the area where it is needed – in the plant root zone, is proposed. In this method, narrow strips of soil centered on the seed line are treated with steam rather than the whole bed.

Over the course of the last year, we developed a prototype band-steam and co-product applicator that is designed to raise soil temperatures in a band 2” deep by 4” wide to levels sufficient to control soilborne pathogens (140 °F for > 20 minutes) and weed seed (150 °F for > 20 minutes). The device is principally comprised of a 35 BHP steam generator and a co-product applicator mounted on top of a bed shaper (Fig.1). The apparatus applies steam via shank injection and from cone shaped ports on top of the bed shaper. An exothermic compound can be co-applied via shank injection and/or a banding spray nozzle. The rationale behind co-applying an exothermic compound with steam is that exothermic compounds react and release heat when combined with water, thereby reducing energy requirements and increasing travel speed.

Preliminary testing of the device this spring in Yuma, AZ were very promising. Trial results showed that application of steam alone effectively raised soil temperature in the center of the seed line to levels required for effective pest control (140 °F for more than 20 minutes). Use of the exothermic compound increased soil temperature by about 10 °F. A video of the device in action can be found at the link provided below.

We are currently evaluating the device in field trials with lettuce in Salinas, CA. Target pests in these experiments conducted in collaboration with Steve Fennimore, UC Davis, are soil pathogens which cause Sclerotinia lettuce drop and in-row weeds. Future articles will report the findings of this research.

This fall, we will be replicating these tests in Yuma, AZ and also investigating the effectiveness of band-steam for controlling Fusarium oxysporum f. sp. lactucae which causes Fusarium wilt of lettuce. Heat has been shown to effectively kill Fusarium oxysporum spores and control Fusarium wilt disease. As an example, soil solarization, where clear plastic is placed over crop beds during the summer, raises soil temperatures to 150-155˚F at the soil surface, effectively killing the pathogen and reducing disease incidence by 45-98% (Matheron and Porchas, 2010). 

These projects are sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council.  We greatly appreciate their support.

If you are interested in seeing the machine operate or would like more information, please feel free to contact me.

See the band-steam and co-product applicator in action!

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.

The volatility of herbicides, or the change from a solid or liquid to a gas, is dependent on several environmental factors and is extremely variable. We have been working on finding a replacement for Glyphosate for non-crop weed control and have tried to determine the stability of the potential  herbicide alternatives. There are various methods used to measure herbicide volatility. All herbicides are initially tested in the laboratory to determine volatility and other properties. Volatility is specifically measured by placing a given volume of herbicide in a container, exposing it to various temperatures and humidity’s and then weighing how much is left. This is done under very controlled conditions. Another technique that is often the next step is to conduct bioassay studies in a greenhouse. This usually involves placing a container with the herbicide spray solution in a closed environment with sensitive plants. Injury to the bioassay plants are measured visually or by some other means.  Field Studies are often conducted to measure herbicide volatility. This technique is the most applied, but the results are often imprecise and variable depending on environmental conditions. This commonly involves spraying an isolated area in the field and after the spray has settled placing sensitive plants at variable distances and directions away. Injury is observed or measured at variable time periods. We used this technique on June 10 to June 15 this year at the Yuma Valley Agriculture Center to measure volatility of 13 herbicides we are evaluating as alternatives to Glyphosate. Seven X 10 Ft. plots were sprayed, and tomato plants were placed 25Ft. away from each sprayed area on the north, south, east and west corners 1 hours after application A 50 Ft. buffer separated each sprayed plot. Visual injury was measured to the tomato plants at 24 and 48 hours after they were placed in the field. The 13 herbicides were used in this trial included 5 modes of action and are listed below.

The temperature reached above 100 F, the humidity was 10 to 20% and wind was 5 to 10 MPH during the trial. No injury symptoms were observed to any of the tomato plants from any of the herbicide treatments. The trial included low volatility formulations of the plant growth regulators, 2-4-D (Embed) and Dicamba (Enginia) which are often volatile under these hot and dry conditions. Neither of these two, or any of the other included herbicides, moved 25 ft or more in this one trial. We know, however, that in other trials the results have sometimes been different. Volatility is variable and difficult to measure in field trials.

Results of pheromone and sticky trap catches can be viewed here.
 
Corn earworm:            
CEW moth increased the Dome Valley area last week, but areawide below average for early October.
 
Beet armyworm:         
Trap counts highest in Tacan, Wellton and Dome, but below average for early fall.
 
Cabbage looper:          
Cabbage looper numbers remained low consistent with late-September.
 
Diamondback moth:   
DBM moths beginning to appear in traps in Wellton/Dome Valley, trending on average for early fall..
 
Whitefly:                      
Adult movement down in the past week, above average for late-September. 
 
Thrips:                         
Thrips adult activity beginning to increase, and trending about average to previous years.
 
Aphids:                        
First winged adults captured for the season, consistent with heavy winds from W-NW last week. Can anticipate aphid flights to increase in the coming weeks.
 
Leafminers:                   
Adult activity tending downward in most locations, average for late-September.