May 5, 2021Summer Sanitation Is Important as Ever
To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
It is much easier to kill weeds when there is no crop in the field and now is a good time to reduce the seed bank of summer annual weeds in fallow fields. Weed seeds are buried at variable depths in the soil, some have hard seed coats and there are other variables that cause them to germinate over a long period of time. If they all came up at the same time they would be much easier to control. It takes time, therefore, to repeatedly irrigate, germinate and kill weeds with either tillage or herbicides. We have conducted trials that indicate that in most years summer annual weeds begin to germinate in February, reach a peak in June but continue to germinate into October.
Proper timing of tillage to kill weeds can be important with some species. Some weeds like common Purslane are very succulent and can remain viable for several days after cultivation or hoeing. They can reroot at the nodes and continue to grow if they are allowed to get too big before they are uprooted. Growers sometimes allow early emerging weeds to get fairly big in an effort to germinate as many seeds as possible. Incorporating large amounts of organic matter into the soil can also have a negative effect on some preemergent herbicides used in vegetables. Many of the root and shoot inhibitor herbicides like Trifluaralin, Pendimethalin, Benefin, DCPA and others can bind to organic matter and be less available to kill weeds.
Tillage has the opposite effect on perennial weeds such as nutsedge and bermudagrass than it has on annual weeds. These weeds are spread vegetatively and repeatedly irrigating and tilling them will spread rather than kill them.
Both contact and systemic herbicides are used during fallow periods to control weeds. The contact herbicides include Paraquat (Gramoxone, Firestorm), Carfentrazone (Aim, Shark), Pyraflufen (ET), Pelegonic Acid (Scythe),Glufosinate (Rely,Liberty) and others. Some of the advantages of these are that they are quick and have no soil residual allowing crops to be planted soon after application. Disadvantages are that they are effective primarily only on small weeds.
The most commonly used systemic herbicide for fallow ground is Glyphosate. It is broad spectrum and has no soil residual. Many of the systemic herbicides registered for fallow use, such as Oxyfluorfen (Goal, Galigan) or EPTC (Eptam) require at least 90 days before planting many vegetable crops. If done correctly, Eptam can be very effective in controlling nutsedge during summer fallow.
Only the fumigants kill weed seeds. These include Chloropicrin, Methyl Bromide, Metam Sodium, Dazomet, Telone and others. Most preemergent herbicides only work after the seed has germinated. Preemergent herbicides are often used for fallow weed control only when at least 30 to 45 days or longer are available. Fumigants are expensive, can be difficult to use and are often used for disease or nematode control with the added benefit of controlling weeds. Unlike soil active herbicides, Fumigants do not have any residual activity.
Soil solarization and flooding have become increasingly popular in recent years as techniques to control pests during summer fallow. Few regions are as well suited for these techniques as the low desert. They are used primarily to control diseases but have the benefit of controlling some summer annual weeds as well. Summer flooding works better here in the low desert than it does in many places because of the high temperatures and high respiration demands. The availability of oxygen is cut off to the roots when it is most needed. It is necessary to keep the field continuously flooded at a depth of 6 to 8 inches for 3 to 8 weeks. Some species are much more sensitive than others to this technique. Perennial weeds are more sensitive than are many annual weeds. Pigweed, field bindweed and nutsedge survive while many annual grasses do not.
Last year we had a lot of watermelon fields infected with Fusarium from Winterhaven to Yuma, Wellton, and Mohawk Valley. Rain, and overwatering of fields when plants set fruits might have contributed to the disease development.
Fusarium wilt of watermelon, caused by Fusarium oxysporum f. sp. niveum, is one of the oldest described Fusarium wilt diseases and the most economically important disease of watermelon worldwide. It occurs on every continent except Antarctica and new races of the pathogen continue to impact production in many areas around the world. Long-term survival of the pathogen in the soil and the evolution of new races make management of Fusarium wilt difficult.
Symptoms of Fusarium can sometimes be confused with water deficiency, even though there is plenty of water in the field. In Yuma valley we have seen fusarium problem in some overwatered fields.
Initial symptoms often include a dull, gray green appearance of leaves that precedes a loss of turgor pressure and wilting. Wilting is followed by a yellowing of the leaves and finally necrosis. The wilting generally starts with the older leaves and progresses to the younger foliage. Under conditions of high inoculum density or a very susceptible host, the entire plant may wilt and die within a short time. Affected plants that do not die are often stunted and have considerably reduced yields. Under high inoculum pressure, seedlings may damp off as they emerge from the soil.
Initial infection of seedlings usually occurs from chlamydospores (resting structure) that have overwintered in the soil. Chlamydospores germinate and produce infection hyphae that penetrate the root cortex, often where the lateral roots emerge. Infection may be enhanced by wounds or damage to the roots. The fungus colonizes the root cortex and soon invades the xylem tissue, where it produces more mycelia and microconidia. Consequently, the fungus becomes systemic and often can be isolated from tissue well away from the roots. The vascular damage we see in the roots is the defense mechanism of the plant to impede the movement of pathogen.
Disease management include planting clean seeds/transplants, use of resistant cultivars, crop rotation, soil fumigation, soil solarization, grafting, biological control. An integrated approach utilizing two or more methods is required for successful disease management.
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!