Western flower thrips are now beginning to increase on produce crops throughout the area. This is evident from sample collected here at the Yuma Ag Center and by the increase in areawide trap catches (see Areawide Trap Insect Network). They have slowly been increasing in the Yuma Valley, and should likely continue to increase as the days grow longer. The recent rainfall we had last weekend may temporarily reduce numbers, but don’t expect that to last very long. Based on historical data, if temperatures remain moderate and rainfall is light we can expect thrips numbers to reach high levels by the mid-March and into April (see graph below). Another factor PCAs should be concerned with this time of the year is thrips "bioconcentration" which occurs each year in March as lettuce acreage declines. This could be especially important this season since the abnormally warm temperatures appear to be pushing lettuce harvests ahead of schedule relative to years past. Each time a lettuce field is harvested and disked, adult thrips populations disperse from these areas into the next available lettuce field. This is generally coincident with our seasonally warm temperatures that are suitable for thrips development. As the number of lettuce acres becomes reduced near the end of the season, this creates a bottleneck effect that concentrates high numbers of thrips adults on the remaining fields under production. This can often make chemical control very difficult, particularly in March, as thrips adults may continually re-infest fields following spray applications. Furthermore, by mid-March when most of the lettuce production is finished, these populations can pose a threat to seedling cotton. Note: the key to preventing thrips from significantly scarring leafy vegetable plants is to prevent immature populations from becoming established. For more information on the management of thrips on desert produce please see the Thrips Control Chart 2015.
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 Disease and Weeds with Band-Steam – Yuma Trials Show Good Promise
In previous articles (Vol. 11 (13), Vol. 11 (20), Vol. 11(24)), I’ve discussed using band-steam to control plant diseases and weeds. Band-steaming is where steam is used to heat narrow strips of soil to temperature levels sufficient to kill soilborne pathogens and weed seed (>140 °F for > 20 minutes). The concept is showing good promise. This past season, three trials were conducted examining the efficacy of using steam for disease and weed control in Yuma, AZ. In the studies, steam was applied in a 4-inch-wide by 2-inch-deep band of soil centered on the seedline using a prototype band-steam applicator (Fig.1). The band-steam applicator is principally comprised of a 35 BHP steam generator mounted on top of an elongated bed shaper. The apparatus applies steam via shank injection and from cone shaped ports on top of the bed shaper.
Trial results were very encouraging as the prototype applicator was able to raise soil temperatures to target levels (140°F for >20 minutes) at viable travels speeds of 0.75 mph. Steam provided better than 80% weed control and significantly lowered hand weeding time by more than 2 hours per acre (Table 1). Results also showed that Fusarium colony forming units (CFU) were reduced from 2,600 in the control to 155 in the 0.75 mph and 53 in the 0.5 mph treatments, respectively (a more than 15-fold reduction). A significant difference in Fusarium wilt of lettuce disease incidence was not found, however disease infection at the field site was low (< 2%) and differences were not expected. At 0.5 mph, fuel costs were calculated to be $238/acre which was considered reasonable and consistent with the values reported by Fennimore et al. (2014).
An unexpected finding was that plants in steam treated plots appeared to be healthier and more vigorous than untreated plots (Fig. 2). This trial is still in progress and it will be interesting to see if this improved early growth translates into increases in crop yield.
In summary, early trial results are showing good promise for use of band-steam as a non-herbicidal method of pest control. We plan on conducting further trials in this multi-year study. If you are interested in evaluating the device on your farm and being part of the study please contact me. We are particularly interested in fields with a known history of Fusarium wilt of lettuce and/or Sclerotinia lettuce drop that will be planted to iceberg or romaine lettuce.
As always, if you are interested in seeing the machine operate or would like more information, please feel free to contact me.
Acknowledgements
This work is supported by Crop Protection and Pest Management grant no. 2017-70006-27273/project accession no. 1014065 from the USDA National Institute of Food and Agriculture, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council. We greatly appreciate their support. Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.
A special thank you is extended to Mellon Farms for allowing us to conduct this research on their farm.
References
Fennimore, S.A., Martin, F.N., Miller, T.C., Broome, J.C., Dorn, N. and Greene, I. 2014. Evaluation of a mobile steam applicator for soil disinfestation in California strawberry. HortScience 49(12):1542-1549.
Click link below or picture to see the band-steam and co-product applicator in action!
Carryover of Vegetable Herbicides to Wheat Grown in Rotation
Almost all the herbicides used on lettuce, cole crops and melons have restrictions on how soon wheat can be planted in rotation after they have been used. Experience has demonstrated, however, that safe intervals can vary considerably based upon many factors and are almost always much longer than they need to be. The most important factors are rate applied, irrigation practices and tillage. For example, when Kerb used to be banded at 2 to 4 lbs. per acre after planting and incorporated with furrow irrigation, it was common to see treated strips across wheat fields which followed. This is uncommon now that lower rates are Chemigated. We still see some Balan injury at ends of fields or in overlaps especially when sudan is planted. Wheat it not very sensitive to Prefar and carryover injury is uncommon.
Corn earworm:
CEW moth counts continued to decline over the past 2 weeks across all locations and about average for this time of the season.
Beet armyworm:
Trap counts decreased in most locations, but well below average for late-October. Most activity in Yuma, dome and Gila Valleys.
Cabbage looper:
Cabbage looper trap counts declined in most areas but increased in the Bard and Yuma Valley. Activity below average for early November.
Diamondback moth:
Adults peaked in Gila and Yuma Valleys and above average in all locations for this time of year. Traps located adjacent to cauliflower and broccoli transplants had the highest trap captures
Whitefly:
Adults remains active in Dome Valley, Wellton, and Roll consistent with melon crops completing harvest, but below average movement for this time of season.
Thrips:
Thrips adult movement increased in most locations last week, particularly in Dome Valley, Wellton, and Tacna. Activity about average for early November.
Aphids:
Aphid movement increased sharply in Bard, N. Yuma Valley and N. Gila Valleys over the past 2 weeks. Activity average for this time of year.
Leafminers:
Adult activity increased sharply in the Gila Valley, Roll and Wellton; about average for this time of season.
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
