Now that the temperatures have finally cooled down, it is beginning to feel like “winter” in the desert. With the cooler temperatures, we’ve seen a steady decline in insect abundance. However, if temperatures pick back up, we would anticipate an increase in insect pressure. Remember: insects are poikilothermic (cold blooded). With that, it is always interesting to look back over this past fall in anticipation of what we might experience this spring. A quick look at both recent and historic data on pest abundance recorded from YAC research plots and the (Area wide Trapping Network) suggests that insect pressure this fall was much lower for most pests compared to last fall. First, whitefly adult numbers collected from our Areawide traps were considerably lower than last year. Furthermore, sticky trap captures of whiteflies placed adjacent to cantaloupe fields from Wellton to Texas Hill, were the lowest we’ve seen since 2008. CYSDV incidence estimated at harvest in cantaloupe fields in these areas was significantly lower than we’ve seen on fall crops in several years. Surprisingly virus incidence was also markedly lower in fields in Wellton and Roll; growing areas that have typically had high rates of virus infection. We’re not sure why our virus and whitefly pressure was so much lighter this year but may be related to the decrease in cotton acreage last summer. Similarly, Lepidopterous larvae (beet armyworm, cabbage looper and corn earworm) population abundance on untreated lettuce at the Yuma Ag Center this fall was the lowest we’ve recorded in 4 years. Populations began infesting plots in early September, and remained steady throughout early October, but began to drop significantly in late-October and through November. Our pheromone trap catches (Area wide Trap Network) also show that moths were much less active in October and November relative to last year. The cooler temperatures, particularly nighttime temps, are likely responsible for this. Based on our AZMET weather station here at YAC, the average ambient temperatures this fall in November was, on average, 5° lower than November last year. Finally, Bagrada bug infestations sampled from untreated broccoli plots at YAC were the lowest we’ve observed since the invasive stink bug first showed up in 2010. As expected, populations appeared in mid- September, but never increased to the high numbers we have typically seen during October. Since mid-October, adults have been difficult to find in broccoli and cabbage plantings here at YAC. Western flower thrips population numbers have been relatively high in some areas, but have dropped recently with the cooler weather. We’ve picked up winged green peach aphids on sticky traps, and can find an occasional colony untreated plots. How these aphids and thrips numbers will translate into potential population pressure in the spring is unknown, but you should anticipate their abundance as usual. If we have a wet winter (El Nino), experience suggests that aphids may be quite abundant, where in contrast, thrips don’t do well in wet growing conditions. Nonetheless, graphics showing recent trends in Whitefly, CYSDV, Lep Larvae and Bagrada abundance and a weather summary can be found at Pest Abundance on Desert Produce and Melon Crops in 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.

Diamondback moth:                

• Adult activity increased in all trapping locations over the past 2 weeks, except in the south Yuma Valley.
• Compared with previous seasons, DBM populations about average for this time of the year.
• DBM captures were highest in Bard, north Yuma Valley and Gila Valley in the past 2 weeks and averaged about 4 moths / trap / night.
• Average moth captures across all traps last week averaged slightly more than 3 moths / trap/ night

Results of pheromone and sticky trap catches can be viewed HERE.

Corn earworm: CEW moth activity increased a bit in the past 2 weeks but remains well below average for late spring.

Beet armyworm: Moth counts increased slightly, but remain very low consistent with seasonal temperatures, and below average for this point in the season.

Cabbage looper: Significant increase in activity in Dome Valley, Gila Valley and Tacna, but moth counts remain unusually low for this time of year, as they have all season.

Whitefly:  No adult movement recorded across all locations and overall low numbers consistent with temperatures.

Thrips: Thrips adult movement beginning to pick up considerably, particularly in Yuma and Dome Valleys. Movement is below average for late March.

Aphids: Seasonal aphid counts down considerably compared with the Feb and Jan.  Counts highest in Bard and Gila Valley.  Below average movement for this time of year. Majority of species found on traps were green peach aphid.

Leafminers: Adult activity up slightly in some locations, but well below average for late season.