May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
Although it is not vigorous or vegetative, Shepardspurse is one of the most widespread and difficult to control broadleaf weeds worldwide. I used to think that it spread when there was more alfalfa here and because it is not controlled with 2,4-DB (Butyrate & Butoxone) but it has continued to spread in vegetable crops. It likely has become worse each year because of its growth habits more than its tolerance to herbicides. It germinates from on or just below the soil surface. Herbicides that move or are placed below the surface often miss it. It is difficult to control with Kerb, for instance, because it leaches easily with overhead sprinklers. The seed is less than 0.1 inch in diameter and moves easily in wind and water. It is very small, and the cotyledon leaves are hardly ever seen. By the time you see it, it is at the 3 or 4 leaf stage. It grows rapidly in a rosette that is low to the ground and often covered by the crop. Herbicide coverage is difficult. It soon puts up a thin seed stalk and several seed pods (“purses”). Unlike many annual broadleaf weeds, it can produce several generations in one season. It can grow year round in many regions but has a difficult time surviving the summers in the low desert.
In the past couple of weeks, the reports of INSV in fields has increased dramatically. INSV has been found in fields in Yuma/Gila Valley, Wellton, Tacna, Roll, and Imperial Valley.
PCAs have reported thrips pressure as low this year and most fields have infection less than 1% but some fields have been reported to have higher incidence. The virus has been detected in direct seeded field as well as transplants imported from Salinas, CA.
Impatiens necrotic spot virus, also known as INSV is a tospovirus closely related to Tomato spotted wilt virus. Infected plants usually have leaves with brown to dark brown necrotic areas. Sometimes the symptoms may be confused with “chemical burn”. As necrosis progresses the leaf browns or die out. Plants infected in early stage may become stunted and die, or become unmarketable.
What makes this virus of high economic importance?
The plants become unmarketable which is the ultimate economic loss. But there are factors that facilitate the virus outbreak.
The first one is efficient transmission by its vector (s). The virus is transmitted by western flower thrips, Frankliniella occidentalis.
If you remember our virus transmission series in past newsletters, thrips transmit viruses in persistent propagative manner. 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. Adult thrips can transmit these viruses only if acquired in the larval stage of development. Larval thrips will feed on a virus-infected plant, pupate, and emerge as a winged adult capable of transmitting the virus. The thrips then will carry the virus for life.
The next contributing factor is host range. INSV infects large number of ornamental and vegetable plants. We are talking 600 species of plants that are susceptible to INSV and thrips love flowers.
While it may not be practical to remove all your ornamentals in fear of INSV, it is definitely practical to monitor thrips population in your field. As the legend says “When in doubt, scout”.
And if you need diagnosis, drop the samples in the clinic! But then if you have immunostrips, you don’t have to make the drive to the Ag Center!
DIY testing: Impatiens necrotic spot virus (INSV)
As visual diagnosis of the virus is confusing and could even be misleading at times, it is very important to confirm a symptomatology via clinical diagnosis.
The good news is there are tools available for quick and easy diagnosis of INSV. You can order the immunostrips from Agdia (https://orders.agdia.com/agdia-immunostrip-for-insv-isk-20501)
The immunostrips cost anywhere from $5-20 depending on how much you buy. They perform better when they stay refrigerated until just before use.
Immunostrips are quick and easy tool to use. The kit comes with a buffer bag and immunostrip.
One band means that the positive control worked which means the system worked. Sometimes you see no bands at all. This means the system did not work and you have to repeat the test.
If you are seeing symptoms in your field please let Bindu Poudel-Ward know via email (email@example.com or text (928-920-1110). Please keep a note of weed species you are consistently seeing in your fields and keep the thrips population under check.
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.
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.
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!
Corn earworm: First significant CEW moth activity since mid-November; particularly active in Dome/Wellton/Tacna areas.
Beet armyworm: Moth counts remain very low consistent with seasonal temperatures, but below average for this point in the season.
Cabbage looper: Slight increase in activity, but moth counts remain unusually low for this time of season.
Whitefly: Adult movement is at seasonal low consistent with temperatures and lack of melons or cotton.
Thrips: Thrips activity beginning to pick up, particularly in Tacna and Yuma Valley. Movement is still below average for February.
Aphids: Seasonal aphid counts peaked in early February and tending down last week. Counts remain high in Gila Valley and Wellton. Above average for this time of year.
Leafminers: Adult activity remains light in most trap locations. Trap counts increasing slightly in the South Gila Valley.