After Bemisia whiteflies invaded the desert in 1991, production of fall melons became very difficult, and acreage declined in many desert growing areas. With the Section 18 registration of Admire 2F in 1993, growers could effectively control whiteflies again and soon began to profitably produce fall melons in Yuma, and central Arizona. Then in 2007, a new challenge emerged for melon growers when Cucurbit Yellows Stunting Disorder Virus (CYSDV) became established throughout the desert Southwest. It’s taken some time, but economic production of fall melons is back on track in Arizona. In the past several growing seasons, whitefly populations during the fall have been lighter than normal, and CYSDV incidence on fall melons had been held below 20% incidence except for the 2021 season when CYSDV incidence increased dramatically even though whitefly abundance was relatively light (see graph below). This occurred due to abandoned watermelons (~200 acres) in the Tacna area that were left untreated all summer. The watermelon acreage was found 1-5 mile southeast (upwind) of several cantaloupe and honeydew fields and had an abundance of whiteflies and heavily infected with CYSDV.  Although the abandoned watermelons had not been irrigated during the summer, heavy monsoon activity in July and August (>3” rain), sustained foliage in the fields and allowed both whitefly adults and CYSDV to build up to unusually high levels. We estimated that the incidence of CYSDV in all the cantaloupe/honeydew fields in the Tacna and Roll areas was greater than 80% at harvest. Melon yields were significantly impacted, and a couple of fields did not harvest.  These results stress the importance of cultural management of whiteflies exemplified by results of the fall 2022 season (as well as the 2015-2020 growing seasons) in the Tacna/Roll growing area. To download the most recent information for whitefly and CYSDV management in fall melons go to 2023 Guidelines for Whitefly / CYSDV Management on Fall Melons. 
 
The epidemiology of CYSDV is a complex relationship between the virus, the adult whitefly vector and our local melon cropping system.  It has been our goal over the past 16 years to understand this pathosystem and develop practical approaches for reducing CYSDV impact on fall melon production. We continue to develop new information on cultural and chemical control tactics for adult whiteflies (see 2023 Guidelines).  Our knowledge to date suggests that fall melons produced near cotton, alfalfa or near areas where spring melons were recently produced are at the highest risk of infection. When practical, growers should attempt to isolate fall melon plantings as far away as possible from these sources of whiteflies and CYSDV. Growers forced to plant fall melons near these crops should be vigilant in minimizing adult whitefly infestation levels with insecticides during pre-bloom growth stages. This can be achieved with at-planting and side-dress applications of soil insecticide (Venom, Scorpion, Sivanto) and well-timed applications of foliar sprays including PQZ and Sefina, feeding disruptors with a high degree of bee safety. Based on local research, both products will provide excellent adult knockdown and CYSDV suppression when used in a fall management program.  Transform, Assail, Minecto Pro and Exirel are also labeled for use on melons and provide rotational alternatives for adult knockdown.  Bottom line: cost-effective chemical tools are available, but minimizing whitefly abundance in the surrounding crop landscape (e.g., cotton, volunteer melons) is critical for them to be most effective.

Soils provide the foundation of crop production systems and there are several chemical properties worthy of our attention (Parikh and James, 2012).  In consideration of the three main categories (physics, chemistry, and biology) describing a soil system in relation to soil health, soil chemical properties represent an extremely important aspect of healthy soil function (Figure 1).

Figure 1.  Soil health factors and chemical properties.

The solid phase of soils is composed of both mineral and organic components.  The mineral portion of the soil consists of sand, silt, and clay fractions.  In this article, the focus is on the mineral portion of the soil system and the basis of soil cation exchange capacity (CEC).  
 
The relative proportions of sand, silt, and clay fractions define the soil texture (Figure 3).  Soil texture is probably the most fundamental characteristic of a soil since it affects many of the important physical, biological, and chemical processes in a soil.  Despite any management practice that is used in a field, the soil texture changes very little over time.

Figure 2.  Soil textural triangle.

One aspect of soil texture that is very important in relation to soil chemical properties is the clay or colloidal fraction, particles less than 0.002 mm or the less than 2 µm (micron) in diameter.  This is important because of the extremely small size of the particles which renders a very large amount of surface area among the clay particles or the soil colloidal fraction (Figure 3).

Figure 3.  Relative size of soil particles.

In the natural process of soil formation and both chemical and physical weathering the secondary clay minerals, layer silicates, are formed.  Fine-grained micas, chlorite, and vermiculite are formed through the basic processes of weathering of primary aluminosilicate minerals.  Kaolinite and oxides of iron and aluminum are products of much more intense or longer duration weathering.
 
Soil composition is a direct reflection of the materials from which they are made.  Parent material for the mineral fraction of soils are normally the primary aluminosilicate minerals.  As the name implies, these minerals are made from aluminum (Al), silicon (Si), and oxygen (O).  Collectively, these elements account for approximately 82 % of the earth’s crust by weight.  On a mass basis, O and Si are extremely important and are dominate components in mineral soils (Table 1).

Table 1.  Average chemical composition of the earth's crust. 
 
In soil minerals, both Si and Al are found as small, highly charged positive cations (Al³⁺ and Si⁴⁺), which coordinate with the larger and negatively charged O anion (O^2-) to form silica tetrahedron and the aluminum octahedron. In both structures, the small central ions are surrounded by either four or six O^2- or OH^- ions in coordination which serve to "hide" the cations and expose the O or OH groups to the soil solution. On a volume basis, the larger anions dominate the soil minerals, both primary and secondary (Brady and Weil, 2008; Magdoff, and van Es, 2021).

Layer silicate clays

These important silicate clays are also known as phyllosilicates (Phyllon - leaf) 
because of their leaf-like or plate like structure. These are made up of two kinds of 
horizontal sheets.  One dominated by silicon and other by aluminum and/or magnesium.
 
Silica tetrahedron: The basic building block for the silica-dominated sheet is a 
unit composed of one silicon atom surrounded by four oxygen atoms.  It is referred to as 
the silica tetrahedron because of its four-sided configuration.  It is based on an interlocking array or a series of these silica tetrahedra tied together horizontally by shared oxygen anions rendering a tetrahedral sheet (Figure 4).

Figure 4.  Layer silicate silicon tetrahedral structure.

Alumina octahedron: Aluminium and/or magnesium ions are the key cations surrounded by six oxygen atoms or a hydroxyl (OH^-) group giving an eight-sided building block referred to as an octahedron.  Numerous octahedra linked together horizontally comprise the octahedral sheet (Figure 5).

Figure 5.   Layer silicate aluminum octahedral structure.

An aluminum-dominated sheet is known as a di-octahedral sheet, whereas one dominated by magnesium is called a tri-octahedral sheet.  
 
The distinction is due to the two aluminum ions in a di-octahedral sheet that satisfy the same negative charge from surrounding oxygen and hydroxyls as three magnesium ions in a tri-octahedral sheet.
 
The tetrahedral and octahedral sheets are the fundamental structural units of silicate clays.  These sheets are bound together within the crystals by shared oxygen atoms into different layers.  The specific nature and combination of sheets in these layers vary from one type of clay to another and control the physical and chemical properties of each specific type of clay (Figure 6 and Table 2).

Figure 6.  One to one clay mineral structure.

Isopmorphous Substitution
In some of the silica tetrahedra, Si⁴⁺ is replaced by Al³⁺ and the charge deficit (a net negative charge) is balanced by monovalent (single charge) or divalent (two charge unit) cations.  
 
Similarly, in the octahedra, Al³⁺ may be replaced by Mg²⁺ or other divalent cations.  This replacement also generates a net negative charge deficit that must be balanced by cations that are not an integral part of the octahedral or tetrahedral network.
 
In the cases of silica tetrahedra and aluminum octahedra, the substituting ions are of similar size.  Thus, this exchange process is referred to as “isomorphous” substitution.  Where “iso” means “same or similar” and “morphic or morphous” refers to the physical size and structure.  This substitution refers to similar sized ion replacement but with different charge characteristics (Figures 7 and 8).

Figure 7.  Isomorphic substitution in a clay mineral structure.


Figure 8.  Two to one (2:1) clay mineral structure (two Si tetrahedrons and one
Al octahedral sheet in each layer) and the van der Waal’s bonds linking or
stacking the 2:1 layers.

Cation Exchange Capacity (CEC)
The result of this ionic substitution is a net negative charge on the surface of clay particles.  The net negative on the soil clay or colloidal fraction provides a reactive surface of clay minerals that interact with cations (e.g., K⁺, Mg²⁺, Ca²⁺, Na⁺, etc.) in soil solution (Figure 8).  
 
Thus, the greater the clay or colloidal fraction in the soil solid phase, the greater the reactive capacity the soil has.  Since the net clay or colloid surface is negative, the reactions with the soil solution involve cations, both mineral and organic.  Thus, this exchange phenomenon is referred to as “the cation exchange capacity, CEC” (Figure 8 and Table 2).

Table 2.  Relationship between soil texture and clay type to the cation
exchange capacity (CEC, meq/100g).

The higher the soil CEC, the greater nutrient holding and exchange capacity of the soil.  This aspect of a soil system also determines the water holding capacity to a very large extent because of the physical/chemical interaction between the soil colloid surface and water molecules (Figure 9).

Figure 9.  Soil-water holding capacity as a function of soil texture. Source: The
COMET program.  University Corporation of atmospheric research.


Figure 10.  A plant root hair extending into soil solution and interacting with
adjacent soil particles and cation exchange.

Healthy soil exhibits a full expression of the solid phase chemical capacity and function, including the cation exchange capacity (CEC), which largely defines the nutrient holding and exchange capacity of the soil (Figure 10).  In this manner, a healthy soil system helps support healthy crops.
 


References:
Brady, N.C. and R.R. Weil. 2008. The Nature and Properties of Soils, 14th ed. Prentice Hall: Upper Saddle River, NJ.
 
Magdoff, F. and H. van Es, 2021. Soil Particles, Water and Air.  In: Building Soils for Better Crops  Sustainable Agriculture Research & Education Outreach Program. Chapter 5.
 
Parikh, S. J. and B.R. James. 2012. Soil: The Foundation of Agriculture. Nature Education Knowledge 3(10):2
https://www.nature.com/scitable/knowledge/library/soil-the-foundation-of-agriculture-84224268/#

Damping-Off
This disease commonly affects seeds and young transplants and is caused by the soil-borne fungi such as Pythium, Fusarium, Rhizoctonia etc. Infected seeds decay in the soil. Seedlings and young transplants will “damp-off” or rot at the soil line, before they eventually collapse and die. 
The fungus, Rhizoctonia solani, causes wirestem. Stems of plants become constricted and brittle at the soil line. The plant becomes stunted and may rot at the soil line. This disease is more severe on fall cole crops when the soil is warm. We have seen lot of this problem in the fields last year. Make sure you get certified disease free seedlings. 

Prevention & Treatment: Cultural controls include planting on raised beds and providing good drainage. In greenhouse where transplants are grown, use new potting soil and new or thoroughly cleaned and disinfested containers and trays. Wash used containers with soapy water to remove all traces of old soil mix, and then briefly submerse containers in a 10% bleach solution. Allow to dry before planting in containers. Both in greenhouse and fields: avoid overwatering and wet feet in plants/seedlings. 

k Rot
Black rot is another common disease we observed in the fields last growing season. Black rot is caused by a bacterium, Xanthomonas campestris pathovar campestris, and can affect all vegetables in the crucifer family. Above-ground parts of the plant are primarily affected, and symptoms may vary depending on the type of plant, age of the plant and the environmental conditions. In general, yellow, V-shaped lesions appear along the tips of the leaves with the point of the V directed toward a vein. When lesions enlarge, wilted tissue expands toward the base of the leaves. Veins turn black or brown. Infection may spread into the stems. Cutting into the stems often reveals a black-brown discoloration with a yellowish slime present. Symptoms on cauliflower may appear as numerous black or brown specks, black veins and discolored curds.

Prevention & Treatment: With no effective curative measures available, preventative measures are very important. The bacteria survive the winter on plant debris and on weeds, such as wild mustard and Shepherd’s purse. It also can survive in and on seeds from infected plants. It can remain alive on plant residue buried in the soil for up to two years. The disease is easily spread by splashing water, wind, insects and garden tools. High temperatures and humidity favor development of the disease.
Use certified disease-free seed and transplants. If source of the seeds is unknown, or infested seedlots must be used, treat seed with hot water to eradicate pathogenic bacteria. Cabbage, broccoli, and Brussels sprouts can be treated at 122 °F for 25 minutes, while seeds of cauliflower, kale, turnip, and rutabaga are treated for 15 minutes. However, this treatment may reduce the viability of seed.
Choose varieties tolerant to black rot.  Do not plant cole crops where black rot has occurred in the past two to three years. Select well-drained sites with good air circulation. 
 
 
Downy Mildew
This disease is caused by the fungus Peronospora parasitica and can attack both seedlings and mature vegetable plants. Infected plants develop a gray mold on the lower leaf surface. The upper leaf surface of infected plants first turns yellow and then may turn brown or necrotic. Leaves wither and die. Symptoms differ from powdery mildew in that the downy mildew fungus grows only on the lower surface of the leaf. Development of the disease is favored by moist conditions.

Article source: https://hgic.clemson.edu/

In a prior article (Vol. 14, Issue 19), I discussed the initiation of a fall 2023 trial examining the use of band steam for controlling Fusarium wilt of lettuce. Again, the premise behind the research is to use steam heat to raise soil temperatures to levels sufficient to kill soilborne pathogens. For Fusarium oxysporum f. sp. lactucae (FOL) the pathogen which causes Fusarium wilt of lettuce, the required temperature for control is generally taken to be > 140°F for 20 minutes. Soil solarization, where clear plastic is placed over the crop bed during the summer, exploits this concept. The technique raises soil surface temperatures to 150-155˚F, effectively killing the pathogen and reducing FOL disease incidence by 45-98% (Matheron and Porchas, 2010).

In our trials, we used steam heat to raise soil temperatures. Steam was delivered by a 35 BHP steam generator mounted on a custom designed elongated bed shaper (Fig. 1). As the device traveled through the field, steam was injected into the soil in narrow bands centered on the seedline. Four band sizes ranging from 4” wide x 2” deep to 6” wide by 4” deep were examined to determine the optimal width and depth of the band of soil that needs to be disinfested to prevent disease. Travel speed was 0.2 mph and soil temperatures were raised to about 190°F. After cooling (< ½ a day), the crop (FOL susceptible iceberg lettuce variety El Guapo) was planted into the disinfested soil.

Preliminary results are very encouraging (Table 1, Fig. 2). The percentage of diseased plants was roughly four times lower in steam treated plots as compared to the untreated plots at the first evaluation date, 39 days after planting (DAP) (19% vs ~ 5%), and about two times lower at the second evaluation date, 60 DAP (32% vs ~17%). These results are consistent with our previous studies which showed that in soils that have moderate FOL inoculum levels, steam treatment reduced disease incidence at maturity by more than 40%. A surprising result was that the width and depth of soil steam treatment did not have a significant effect on disease incidence. A logical explanation for this result could not be formulated as one would expect disease incidence to be reduced if a larger volume of soil is disinfested.

Stay tuned for final trial results!

On a slightly different note, if you are interested in seeing the plots, I will be at the field site Field Day this Wednesday, November 30^th from 8:00 am-12:00 pm. The location is the JV Farms Fusarium Trial site, located off of Hwy 95 and Ave 5E (32.6920898, -114.5395051).

Hope to see you there!


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.

Acknowledgements
This project is sponsored by USDA-NIFA, the Arizona Specialty Crop Block Grant Program and the Arizona Iceberg Lettuce Research Council.  We greatly appreciate their support.

A special thank you is extended to Fatima Corona, Pablo Delgado, Chad VanMatre, Matt McGuire and JV Farms for their generous assistance and allowing us to conduct this research on their farm.
 

Fig. 1. Band-steam applicator principally comprising a 35 BHP steam generator
mounted on a bed-shaper applicator sled. The device injects steam into the soil
as beds are formed.


Fig. 2. Fusarium wilt of lettuce control with band-steam in iceberg
lettuce. Steam was applied in a narrow band (4” wide x 4” deep)
centered on the seedline prior to planting. After the soil cooled (< 1/2
day), the crop was planted into the disinfested soil.

These types of herbicides are also called contact herbicides. PPO inhibitor means they slow down the production of the protoporphyrinogen oxidase (PPO) enzyme, which is used in chlorophill production. This inhibition also results in the formation of highly reactive molecules that attack and destroy lipids and protein membranes which ultimately affect cell membranes. This result in leaky cells and disintegration of the tissue.

Goal (Oxyfluorfen) is one of the earliest registered of these products (1980) but has only been used widely over the about the last 16 years. This is the only one of these that has problems with “lift off” or codistillation with water. ⁽¹⁾
According to University of CA IPM “In dry bulb onions, GoalTender may be tank-mixed with other pesticides such as bromoxynil, but tank mixes must only be used on older onions with well-developed cuticles to avoid unacceptable crop injury. Unacceptable injury may occur if applied to small onions without adequate cuticle development, and in conditions such as after cloudy or rainy weather. Follow the label closely to avoid crop injury when tank mixing.
Oxyfluorfen has some residual soil activity after application. It controls small broadleaf weeds, some grasses, and nightshade, and controls little mallow (cheeseweed) well ”. ⁽¹⁾

Other PPO herbicides are: Aim or Shark, Chateau, Sharpen or Treevix, ET, and Spartan. They work as contact herbicides and do not move through the plant.
Most of these products don’t have preemergence activity but when used at higher rates Goal and Chateau do.⁽²⁾ As the plant emerges from the soil and contacts the herbicide it dies, so it is recommended to avoid disturbing soil after the application of these products.
 
This week in Yuma oxyfluorfen uses:

1. Directed application in broccoli for goosefoot, lambsquarter, and cheeseweed control.
2. Broadcast application in onions for goosefoot, grasses and lambsqurter

 
References

1. https://cales.arizona.edu/crop/vegetables/advisories/more/weed173.html
2. https://ipm.ucanr.edu/agriculture/onion-and-garlic/integrated-weed-management/

Area wide Insect Trapping Network   VegIPM Update, Vol. 15, No. 21, Oct 16, 2024

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

Corn earworm: CEW moth counts increased in most traps; about average for early October.

Beet armyworm: Trap counts increased in most areas last week, and about average for early October.

Cabbage looper: Cabbage looper trap counts still most locations, below average for this time of the season.

Diamondback moth: Adult activity increasing, particularly in the Yuma and Dome Valleys.  Above average for this time of season.

Whitefly: Adult movement increasing Tacna, Dome Valley and Yuma Valley; overall about average for early October.

Thrips: Thrips adult movement picking up in all areas, overall activity above average.

Aphids: Winged aphids beginning to show up the north Yuma and Gila Valleys; about average for early October.