It’s that time of the year again. Desert growers have begun planting fall melon crops, transplanting cabbage and will be direct seeding leafy vegetables in just a few weeks. Soon to follow is a number of important insect pest issues. As crops begin to emerge, PCAs will begin to encounter insects that have the potential to cause serious economic losses to seedling crops during stand establishment. These include flea beetles, crickets (sometimes grasshoppers), darkling and rove beetles, and saltmarsh caterpillars. These insects all have chewing mouthparts and most are capable of consuming large amounts of leaf tissue in a short time. Seedling crops at the cotyledon stage are most susceptible to these pests, where feeding by large numbers can devour much of the cotyledons or outright kill the small plants. If left uncontrolled, larger seedling plants (2-4 leaf stage) can sustain significant feeding damage on the terminal growing points or newly emerged leaves. Not only can this feeding stunt plant growth, but it can result in lack of uniformity and maturity at harvest. Host sources of flea beetle, cricket and "woolly worm" infestations include numerous summer crops (e.g., sudan grass, cotton and alfalfa) and a large host of weeds (e.g., purslane). Experience indicates that melon fields planted adjacent to these crops/weedy areas are at a high risk from these seedling pest, and particularly flea beetles. As summer crops are harvested or terminated during the next several weeks, these seedling pests typically move to the next available host crop; lettuce, cole crops and melons. Fortunately, there are many registered insecticide alternatives available that can be applied via sprinkler chemigation (i.e., pyrethroids) or foliar sprays (i.e., Lannate, neonicotinoids) that can cost-effectively minimize their abundance and damage to emerging produce and melon crops. For more information on insect pests of leafy vegetables and melons at stand establishment go to the document Pests at Stand Establishment 2014.

As reductions of agricultural water allocations in central Arizona from the Colorado River are going into place in 2022 from the Tier 1 of the Drought Contingency Plan provisions, it is important for every segment of the Arizona agricultural industry to continue to review and refine our practices associated with water management and irrigation.  
 
Seasonal conditions are a benefit to vegetable crop production and water management in the desert Southwest, both in terms of cool-season crop adaptability and the general climatic conditions that create lower environmental crop water-use demands.  For example, based on more than 30 years of Arizona Meteorological Network (AZMET) data from the Yuma, Arizona area, approximately 20% of the total annual reference evapotranspiration demand typically occurs between August and May.  
 
Reference evapotranspiration (ETo) is the collective loss of water from a field due to evaporation from the soil surface and transpiration of water vapor from the plants.  Collectively, water movement from the soil and plant systems in a field is referred to as “crop consumptive use” and it is important to understand this process in our efforts to steward our water resources most effectively.
 
The AZMET system provides both historical and real-time weather information that can be used to assist in crop water and irrigation management.  Reference evapotranspiration values can be obtained daily from AZMET for the Yuma area and applicable to most of the region in the lower Colorado River Valley.  The ETo values multiplied by an appropriate crop coefficient (K_c) can provide very good estimates on actual crop evapotranspiration (ET_c) rates, ETo * K_c = ET_c.
 
The appropriate K_c values are specific for each crop species and stage of growth.  We commonly use crop coefficient K_cvalues that are provided in the publication “Consumptive Use by Major Crops in the Desert Southwest” by Dr. Leonard Erie and his colleagues, USDA-ARS Conservation Research Report No. 29 (see attached).  Reference information for K_cvalues can be obtained in this publication for common vegetable crops grown in this region.
 
Estimating and tracking actual crop-water use can be a valuable tool in understanding crop-water demand and irrigation management.  Comparing actual crop-water demand and crop irrigation practices can be important in our efforts to understand crop management needs and field-level efficiencies.
 
Reference:
Erie, L.J., O.A French, D.A. Bucks, and K. Harris. 1981. Consumptive Use of Water by Major Crops in the Southwestern United States.  United States Department of Agriculture, Conservation Research Report No. 29.

Viruses vectored by insects 
 
We are on the final section of virus transmission. Virus transmission by insects is one of the most efficient and economically important transmission in agriculture. When you have insects in your crops, not only you are losing your crops because of feeding/chewing by insects, a lot of insects also act as a vector of plant viruses. 
 
Seven out of 29 orders of insect feeding on living green land plants are vectors of plant viruses. 
Insect transmit viruses in 4 distinct modes: 
 

1. Non persistent transmission: The insects can acquire the virus in a matter if seconds/minutes and they are immediately viruliferous. The virus in retained in the stylet of the insect and are transmitted to the next plant the insect feeds on. The virus is retained in the vector only for few minutes and is lost after insect molting. Most viruses transmitted by aphids are non persistent. So when you see few aphids in your melon field and see cucumber mosaic virus symptoms 1-2 weeks later in your field, don’t be surprised. Aphids are efficient vectors, and since viruses are systemic it takes anywhere from few days to 2-3 weeks for the plants to show symptoms. Thus it is very important to manage insects in the field even if you don’t think the ‘pressure’ is not as high. 

Viruses transmitted in non-persistent manner:
 Papaya ringspot virus 
Zucchini yellow mosaic virus 
Watermelon mosaic virus

1. Semi-persistent transmission: The insects can acquire the virus in minutes/hours and there is no latent (incubation) period in the insect. The virus can stay in the insects foregut for hours and is lost after insect molting. Some species of aphids and whiteflies fall in this category.  Example: Cucurbit yellow stunting disorder virus in melons transmitted by whiteflies. 

1. Persistent circulative: 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 for weeks. Virus can be present in the vectors hemolymph but there is no multiplication of virus in the insect body. Vectors in this transmission includes: Aphids, leafhopper, whiteflies, treehopper. 

Example: Beet curly top virus transmission by beet leafhopper 
 

1. Persistent propagative:  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. Tomato spotted wilt virus is transmitted on persistent propagative manner by 9 different species on thrips. 

 

Save the Date : 2024 Plant Pathology Workshop 
When: August 29^th 8AM-12 PM ( breakfast and Lunch provided by Gowan Company and BASF) 
Where: Yuma Ag Center, 6425 W 8^th Street 
What will covered: Plant Pathology program Updates, past season field trial results (we
have some exciting results to share), Q&A to help better Plant pathology program,
Industry panel discussion for all your industry related questions! See you in few weeks! 

Automated thinning machines were first commercialized in 2012, almost a decade ago. Since then, the machines have improved tremendously, and automated thinning has become the standard practice in the Yuma area. As such, I was a bit surprised to see numerous fields this year with sub-optimal thinning performance. Many fields had numerous amounts of excess seedlings that should have been thinned and needed to be removed during the follow-up hand weeding/double removal operation (Fig. 1). There were also some fields where the machine was apparently not readily detecting seedlings, resulting in long sections of plants being killed and a poor final stand (Fig. 2).

Automated thinning machines can and do perform very well when set-up and calibrated correctly. However, field conditions such as lighting, levels of crop residue, weed pressure or soil type (color) change throughout the day and affect the way the machine chooses which plants to selectively thin.

As such, it may be beneficial to establish a protocol where automated thinning machine settings are checked at regular intervals throughout the day to ensure the unit is performing optimally. Doing so may help lower follow-up hand weeding/double removal costs and minimize the number of unnecessary skips in the field.

Fig. 1. Labor crew removing excess lettuce seedlings and weeds after machine thinning.


Fig. 2. Poor final stand establishment following machine thinning, possibly due to incorrect machine calibration.

Preemergence herbicides can be very effective in the low deserts where weeds germinate with every irrigation or rainfall. It is essential, however, that they be at the right place in the soil at the right time. The right place is in the soil around the weed seeds and the right time is when they are germinating. Only fumigants kill seeds. For most other herbicides, the seed must germinate and are then killed. When weed seeds germinate is dependent on a great number of interdependent variables.  These include weed species, soil type, moisture, soil temperature, condition of the seed, depth, organic matter and many other variables. Many seeds have biochemical mechanisms that allow them to germinate at different times. Some may germinate in a few hours and some may not germinate for several years. If they all germinated at the same time, they would be easier to control. The methods used to predict when weed seeds will germinate vary from very sophisticated numeric equations to Farmers Almanac type lore involving things like the thickness of gopher fur, the cuticle thickness of plants or the condition of bird feathers. Some methods will give you a precise day while others will give you just a general idea. In general, as the precision goes up, the accuracy goes down. Regardless of what technique you use, now is the best time to apply preemergence herbicides for the control of annual weeds. It is better to be a month early than a day late.We have conducted trials to determine how long it takes various weed species to germinate after receiving moisture. Results will vary depending on several factors and from field to field and year to year. Results are summarized below for individual species and by month. An average of all species is also included.
   Hours to Germination

NG = No germination

Corn earworm: Moth activity has declined in most areas and is about average for early-November. 

Beet armyworm: Moth activity has declined in most trap location and remains below average for this point in the season.

Cabbage looper: Cabbage looper activity remains unusually low for early November.  Trap catches picked up a bit in Wellton and Gila Valley.

Whitefly: Adult movement has been about average for this time of year. Activity highest in Wellton near fall melons being harvested.

Thrips: Thrips activity picking up significantly in several trap locations; activity increased significantly in Bard and Dome and Gila Valleys.  

Aphids: Aphid numbers peaked last week in many locations, particularly in the Gilas and Yuma Valleys.

Leafminers: Adult activity increased significantly last week, particularly in the Wellton area near melons.