Prior to the cold snap that occurred in the low desert last week, green peach aphid populations were steadily colonizing untreated lettuce plots here at the Ag Center. Very few winged forms were found, but small colonies of 5-6 nymphs could be found on about 10% of the plants. However, temperatures in the Yuma Valley dropped below 32° F several nights during that period. Past experience has shown that aphid populations tend to significantly slow down under these “cold” desert temperatures. That is essentially what we observed this weekend; population growth had stalled out and aphid numbers were the same as they were 2 weeks prior. However, past experience has also shown us that aphid population growth begins to increase again once the weather warms up. Our local forecast suggests that temperatures will be in the mid-70’s for the next 10 days. Assuming the weather service is correct, these conditions may be ideal for aphid population growth. Thus far, in the Yuma Valley we have been finding mainly green peach aphid nymph colonies, and we have been picking up winged aphids on our yellow sticky traps. However, we have found winged Foxglove aphids in our celery. As temperatures increase foxglove may become more abundant (they tend to be more biologically active under warmer temperatures; that is when avg. temps > 60 F). If the field has been treated with imidacloprid at planting, then chances are you are in pretty good shape. But, continue to monitor as imidacloprid doesn’t generally last all season, and the neonicotinoids are marginally effective against foxglove and lettuce aphids. Regardless of whether you find green peach aphid or foxglove aphid the key to effective aphid management with foliar insecticide is to initiate sprays at the time apterous (wingless) aphids begin to colonize. Of course, this requires diligent scouting and sampling. On older lettuce, make sure you thoroughly examine developing heads/hearts. Fortunately, PCAs have alternatives for foliar aphid control. For more information on insecticide alternatives and Aphid Identification please visit Aphid Management in Desert Produce Crops – 2015 andAphid Identification in Desert Produce Crops So keep your eyes open for new aphid colonies and plan accordingly. As Thomas Edison once said “Good fortune is what happens when opportunity meets with planning”.

Whenever we go to the field to evaluate a crop, there are typically three fundamental points we need to take into consideration that include: 1) stage of growth, 2) general crop vigor and condition, and 3) anticipate the next stage of development and what we need to do in managerial response.  These considerations are commonly focused on aboveground crop evaluations.  However, the root system, completely belowground, is an extremely important part of the crop condition to evaluate.
 
Root systems are responsible for all water and nutrient uptake by the plant and provides the physical anchoring and support of the plant structure.  Each plant and crop species has its own “personality” and growth habits and the nature of the root systems also vary among plants species.  Many plant species can have root systems that constitute ~ 50% of the total plant dry matter and we generally consider plants to have at least 30% of total plant dry matter allocated to the root systems.
 
Young plants will often develop some basic aboveground tissue to perform sufficient photosynthesis for establishment and then devote a high amount of the energy, carbohydrates produced, along with the nutrients and water from the soil to root system growth and development. Sometimes it can appear that plants are not growing rapidly while investing energy and resources into root system development.  The depth of the roots will vary according to the soil physical conditions and effective soil depth, soil fertility and salinity management, plant-available water, and of course the natural rooting characteristics of the plant.  
 
The effective root zone depth is the depth of soil used by the bulk of the plant root system to explore a soil volume and obtain plant-available moisture and plant nutrients.  Effective root depth is not the same as the maximum root zone depth. As a rule of thumb, we commonly consider about 70% of the moisture that is taken up by the root is obtained in the top half of the root zone; about 20% from the third quarter; and about 10% from the soil in the deepest quarter of the root zone.  The small and very fine root hairs are the most physiologically active portion of a developing root system.  It is important that the plants continue to develop and generate fresh young roots and an abundance of fine root hairs to maintain water and nutrient uptake.
 
Therefore, in scouting fields and making crop evaluations, examining the root systems is an important part of the process.  Leafy green vegetable crops need to develop a marketable plant in a relatively short amount of time and a strong root system is essential.  It does take more time and effort to check root systems and it is a plant destructive process since we need to literally excavate the roots.  So, it is also important be careful of where and how we sample plants and the root systems in a field.

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! 

Mark C. Siemens Vol. 12, Issue 20, Published 10/06/2021 Plans are shaping up for the 2nd Automated Weeding Technologies Field Demo Day. The event will be held Thursday, October 21st at the Yuma Agricultural Center. Registration begins at 7:00 am and the program starts at 7:30 am. We have eight presentations/field demos scheduled (details below). The focus this year is on the latest automated weeding technologies that are “new” since our 2018 event. We would love to showcase as many innovations as possible and it’s not too late to be added to the program. It’s an open invitation - private companies, university and government researchers are all welcome to show their technology. Please contact me if you are interested or know someone that is.

Because there are so few selective herbicides registered for use in lettuce, it is necessary to kill as many weeds as possible prior to planting. Fumigants, flooding and solarization will kill weed seeds but it will be necessary to germinate them first for most herbicides to be effective. Almost any herbicide can be used preplant that is registered for “fallow” conditions as long as the preplant and preharvest intervals are observed. The following table summarizes the most commonly used herbicides for ground preparation in lettuce.

Results of pheromone and sticky trap catches can be viewed here.
 
Beet armyworm:               
CEW moth increased the Tacna/Roll area last week, but areawide below average for mid-September.
 
Beet armyworm:         
Trap counts about the same in all locations, below average for mid-September.
 
Cabbage looper:         
Cabbage looper numbers remained low consistent with mid-September.
 
Diamondback moth:   
DBM moths beginning to appear in traps in Tacna/Dome Valley.
 
Whitefly:                     
Adult movement increased in the past week, consistent with cotton defoliation and harvest. Above average for mid-September. 
 
Thrips:                           
Thrips adults increased in most locations, and trending above average to previous years.
 
Aphids:                        
Aphid movement is absent and consistent with what we typically see during the summer and September. 
 
Leafminers:                   
Adult activity tending downward in most locations, below average for mid- September.