May 4, 2022Spider Mites on Spring Melons 2022To contact John Palumbo go to: jpalumbo@ag.Arizona.edu
In Arizona agriculture, we have the benefit of generally working with good soils that exist in alluvial valleys or the terraces immediately adjacent to the alluvial valleys, e.g. the mesa areas. Arizona soils are geologically young and fertile but often have high levels of salinity and often sodicity. When reclaimed and properly managed with adequate leaching, we can reduce the salinity to manageable levels to support crop production systems. In the case of sodic conditions, appropriate amendments are needed then followed by adequate leaching.
In the process of applying an irrigation in the field, it is important to recognize that not all soils are created equal. Soil types vary across the landscape and they also vary by depth for any site or location. This is particularly true for alluvial soils which originate from water deposition over time, such as from the Gila and/or Colorado River systems. The soils of the lower Colorado River valleys are great examples of alluvial soils and the high degree of variability we commonly experience in the field. With some crops, particularly more deeply rooted crops, we can sometimes nearly map the soil types across a field based on crop growth patterns. Accordingly, this type of soil variability creates some challenges for in-field management, including irrigation management.
Many of the rotation crops common to the lower Colorado River Valleys, such as cotton, wheat, and sudan; are excellent examples of crops that can express growth patterns as a function of soil texture, which is clearly demonstrated in response to water stress. The courser textured parts of the field will stress earlier and consistently have reduced plant vigor. Anyone driving a tractor for medium to heavy tillage operations in the field will literally feel soil textural changes and anyone harvesting those fields will see it as well. The GPS field mapping systems can detect and record these areas of soil type differences through yield monitors as well in response to crop growth and vigor.
Soil textures vary in terms of water holding capacities and it is important to understand the dominant soil textures in the field, not only on the surface but also through the depths of the soil profile and the effective rooting depth of the crop, Tables 1 & 2 and Figure 1. To manage a complete field or set of fields, it is necessary to determine a functional “average” of soil texture and water holding capacity.
In the process of irrigation, we are attempting to replenish the soil-water extracted by the crop through evapotranspiration (ETc). In previous articles, the determination or estimation of crop ETc has been discussed.
Therefore, with irrigation management it is important to know the fields we are working with in terms of the dominant soil textures present, the degree of variability that exists, and the general water-holding capacity of the soils. Matching irrigation timing and volumes for each event to replenish the plant-available water for each field is important in our efforts to avoid water stress and achieve and maintain irrigation efficiency agronomically, which is providing the amount of water necessary to replenish the soil-water to field capacity with some degree of additional water needed for the leaching of soluble salts.
With the high degree of variability that is common among soils in the lower Colorado River Valleys, it is both important and challenging to know the soil characteristics common in each field, the water holding capacity of the dominant soils, and the level of soil-water depletion that is being replenished with each irrigation event.
Table 1. Soil texture and water holding capacity.
Table 2. Depths to which the roots of mature crops will deplete the available water supply when grown in a deep permeable, well-drained soil under average conditions. Source: Chapter 11, "Sprinkler Irrigation," Section 15, Natural Resources Conservation Service National Engineering Handbook
Figure 1. Soil volume, soil texture, and water holding capacity relationships.
Downy mildew has always been one of the major problem for PCAs and growers in the desert southwest. The symptoms observed are green to yellow angular spots on the upper surface of the leaves and fluffy growth on the lower side (See Picture). Symptoms usually start from older leaves. As disease progresses the lesion turn brown and dry up and in some occasions, the disease can become systemic causing dark discoloration of vascular tissue.
Favorable condition for disease development:
The pathogen Bremia lactucae thrives in damp, cool condition, with moisture present on leaves. Spores are short-lived but dispersed efficiently by wind during moist period. Cultivated lettuce is the main host of the pathogen but it has also been reported to infectartichoke, cornflower and strawflower.
Why is downy mildew difficult to manage?
One of the main reason that hinders the disease management is the complexity of the pathogen. Bremia lactucae consists of multiple races (pathotypes), and new races continue to occur as pathogen evolves. The pathogen is one of the fastest evolving plant pathogen. And each pathotypes have developed insensitivity to fungicides to different extent.
One of the best practice is to grow resistant cultivar, but there are limitations. As the pathogen is highly variable and dynamic, resistant cultivars are not a permanent solution as the pathogen overcomes the resistance by evolving into virulent strains and isolates.
Preventative application of fungicides are effective to some extent. Reducing leaf wetness and humidity by using drip or furrow irrigation can be helpful. However, weather condition like rain during cool weather as we had in past couple of weeks is conducive to development of epidemics and we have very little control on that matter.
It Takes a Village:
And better yet the whole nation or world! Downy mildew is a bigger problem than we think. It is just not a problem in Arizona and California, it is a nationwide, worldwide problem. Thousands of plant pathologist/scientists/labs are working hard everyday to combat the disease.
Dr. Michelmore’s lab in University of California-Davis has been working on downy mildew for years and they need our help to build the database for next several years. Please let me (Bindu Poudel) or anyone in University of Arizona know if you see symptoms in the field. We can come collect the samples and send it to UC-Davis. The goal is to racetype as many isolates as possible to understand the genetic variability, the more information we have about the pathogen, the more it helps the scientific community come up with better management practice, better resistant cultivars etc.
Learn more about the Bremia Project:
Bremia Project Database:
Vol. 13, Issue 2, Published 1/26/2022
Public health and environmental concerns about the use of herbicides in addition to the rise in the number of herbicide resistant weeds has led to increased interest in non-chemical methods for weed control. Steam application is one technique that is gaining popularity as an alternative to applying glyphosate and other herbicides in public areas such as parks and school yards. Steam kills weeds by heating plant tissue to levels sufficient to rupture plant cell walls. As you might imagine, the method is energy intensive and slow. To kill even small weeds, exposure to superheated steam for several seconds (2-4) is required (Upadhyaya et al. 1983).
Steam is also used to control weeds in commercial agriculture. There are several manufactures of equipment designed for use in orchards, vineyards and even row crops (Fig. 1-2) (M.M. S.r.l., Modena, Italy; WeedTechnics, Terry Hills, NSW, Australia). These types of machines are equipped with high powered steam generators (200 HP) that convert water to steam at a rate of about 170 gal/hr. Recommended travel speeds are 1-2 mph depending on weed species and density. For orchards and vineyards, steam is typically applied in bands using steam hoods mounted on arms that extend to the crop row (Fig 2).
Despite being organically compliant and indifferent to herbicide resistant weeds, use of steam for post emergence weed control in commercial agriculture has seen limited adoption. A major limitation is that steam provides only partial weed control. Large, susceptible weeds die back, but tend to regrow 3-4 weeks after application and follow up applications are required. Further, some weed species are tolerant to steam when applied at reasonable travel speeds and rates. Abdulridha et al. (2019) investigated the use of steam for weed management in citrus. At travel speeds of 1 mph, goatweed and sedges were controlled (>72%), but Florida pusley and guinea grass were not (< 36%). Other researchers reached similar conclusions; although steam treatment is effective on small weeds (2 true leaves or smaller), control of large weeds is highly dependent on weed species with mortality rates ranging from 100% to 0% (Merfield et al. 2017).
Except for some very specific circumstances, use of steam for post emergence weed control is probably best left to urban areas where cost and time required for application are not as critical as they are in commercial agriculture.
Abdulridha, J.J., Kanissery, R.G., McAvoy, C.E. & Ampatzidis, Y.G. Evaluation of steam application for weed management in citrus. Appl. Eng. in Agric.,35(5): 805-814.
Merfield, C.N., Hampton, J.G. & Wratten, S.D. 2017. Efficacy of heat for weed control varies with heat source, tractor speed, weed species and size. New Zealand J. Agric. Res., 60(4): 437-448.
Upadhyaya, M.K., Polster, D.F. and Klassen, M.J. 1993. Weed control by superheated steam. WSSA Abstracts, 33, 115.
Fig. 1. Applying steam post emergent weed control. (Photo credits: Weedtechnics).
Fig. 2. Applying steam to organically control weeds in a vineyard. (Photo credits: M.M. S.r.l).
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.