- Editor: Laurie Askew
- Author: Rob Wilson
The recent thunderstorms, rain, and overcast conditions have been a huge benefit for reduced irrigation demand and drought relief. On the flip side, these environmental conditions have created favorable conditions for early season late blight development, volunteer potato establishment, and summer annual weeds. Below is a list of spring management activities I encourage farmers to adopt to reduce the potential of late blight reoccurring in 2016.
1.) It is extremely important for growers to scout fields infected with late blight last year for volunteer potatoes, alternative host weeds such as nightshade, and late blight lesions on volunteer potatoes and nightshade. If infection is found, please report it to your cooperative extension office, local PCA, and county agriculture office ASAP so an alert can be posted. I scouted a few grain fields yesterday and I did NOT find late blight lesions on volunteer potatoes but late blight infection is a possibility. Hopefully, the weather turns hot and dry next week.
2.) Eliminate sources of inoculum in cull potatoes and waste potatoes. Methods of disposing cull and non-usable potatoes include feeding to livestock, burial, spraying with herbicides, or composting.
3.) Control volunteer potatoes and nightshade weeds! All growers should try to eliminate volunteer potatoes and nightshade in rotational crops, fallow fields, and field borders. This is extremely important in fields with late blight infection last year and fields that are directly down-wind from fields that had late blight last year. Cultivation and broadleaf herbicides are the most common methods for treating volunteer potatoes and nightshade weeds. Volunteer potatoes are more difficult to control than annual weeds, thus growers should follow the rate recommendations on herbicide labels for volunteer potato suppression. Several small grain herbicides with dicamba and fluroxypyr have label instructions for volunteer potato suppression in small grains.
4.) Scout 2016 potato fields regularly for late blight lesions especially during and after rainy, overcast weather and wet field conditions.
5.) Educate employees how to identify late blight symptoms and develop a preventative management plan for late blight.
6.) Avoid 12 hour irrigation sets and very frequent irrigations. Prolonged leaf wetness of 8+ hours is favorable for late blight infection!
- Author: Rob Wilson
- Editor: Laurie Askew
The warm weather this week is hopefully the best remedy for jump-starting many alfalfa fields damaged by frost and hail the last couple of weeks. If you have fields with frost damage that are not improving, you might dig some alfalfa plants and look at the roots.
Recently we have multiple alfalfa samples brought into the office with damaged roots from clover root curculio (Sitona hispidulus). Clover root curculio has been found in numerous alfalfa fields over the last 20 years, but the infested acreage appears to be increasing and damage is pretty severe this year. Clover root curculio adults are similar in appearance to alfalfa weevil adults, but they are about one-third smaller and have a shorter, blunter snout. Clover root curculio adults also have a mottled brownish coloration on their back compared to the dark brown stripe on alfalfa weevil adults. The adults feed on alfalfa foliage. The white grublike larvae are roughly a ¼ inch long with a dark brown head.
The larvae can be found in the soil feeding on alfalfa roots.The easy way to determine if you have clover root curculio in your field is to look for larvae feeding damage on the roots (see picture).
Larval feeding on alfalfa roots can create points of entry for several alfalfa diseases such as Fusarium and Rhizoctonia. This spring, multiple infested fields had diseased patches where alfalfa was wilted to the ground. One sample was sent to UC Davis for disease diagnosis and it came back positive for Fusarium. Clover root curculio adults overwinter and become active when temperatures exceed 50°F in the spring. Adults lay eggs at the base of alfalfa plants in the spring and larvae hatch and feed on the roots. Larvae pupate in June or July and second generation can sometimes feed on alfalfa in late summer and fall.
Unfortunately, there are not any insecticides registered to control clover root curculio in alfalfa. We plan to do some insecticide testing this year to hopefully find an effective insecticide. Current management recommendations include rotating infested fields into a non-host crop, avoid planting new alfalfa next to infested fields, and proper irrigation and nutrient management. Also make sure to clean to equipment after visiting infested fields to prevent spreading the pest to new fields.
- Author: Steve Orloff
- Author: Larry Godfrey
- Author: Kevin Goding
- Editor: Laurie Askew
Farming sure can be challenging. I guess that is what keeps it so interesting…but a little less interesting might be good sometimes. Last year Klamath Basin alfalfa growers had to deal with blue alfalfa aphid (BAA) and the associated cost of insecticide sprays as well as the yield loss. Fortunately, aphids have not been a problem this year. This may be due to the relatively wet year we have had (especially this spring) providing more favorable conditions for fungi that can keep aphids in check.
While aphids have not been a problem, this has been an incredible year for alfalfa weevil (especially in Scott Valley). Over 100 larvae per sweep have been reported in some fields. In addition to the high populations, emergence has been staggered, making control difficult. It was possible to see weevils at all growth stages in a field at the same time, including overwintering adults, all four instars (larval growth stages) and the new pupating adults from this year's population (Figure 1). This situation makes weevil control especially challenging because they are feeding on fields for such a long time.
Scott Valley growers treated, but there have been numerous complaints about poor control that could not be explained by prolonged weevil emergence alone. It appeared that these fields had poor initial control after treating not just a lack of long-term control. Together with Extension Entomology Specialist Larry Godfrey we have a weevil control trial this year in a grower field where we are evaluating 20 different insecticides and insecticide combinations. While working on this project, we decided to sample commercial fields where growers have experienced poor control after an initial application of Baythroid and a follow-up application with Warrior (Figure 2).
Both of these products are in the pyrethroid class of insecticides and we wanted to determine if the weevils had developed resistance. Four conventional fields were sampled as well as an organic field where insecticides have not been used for decades. The weevils were taken to UC Davis where they were treated in a lab with either Baythroid or Warrior at five rates (1/4 rate, ½ rate, the full field rate, twice the full rate and four times the full rate). The weevils were observed and the average percent mortality calculated. See table below.
Acceptable control levels only occurred for the weevils collected from the organic field that had never been treated with a pyrethroid insecticide. One hundred percent control was not even achieved with weevils from that field perhaps likely due to migration of weevils from treated fields. Control of weevils from fields with a history of pyrethroid insecticide use was extremely poor. At the standard use rate control varied from only 3 to 15 percent. Even at four times the use rate, control was only 23% for weevils collected from three of the fields and 35% in the other.
So…what does this mean? Growers in Scott Valley must rotate to an insecticide with another mode of action, and growers in other parts of the intermountain area should as well if they have been using pyrethroid insecticides alone repeatedly. Unfortunately, there are not many effective insecticides to choose from. Choices include:
- Lorsban -- Category I pesticide with regulatory issues which has caused foliar burn in the Klamath Basin under some weather conditions
- Steward – Significantly more expensive and needs favorable weather conditions to be effective. (In the Low Desert reduced rates of Steward are combined with dimethoate with good results.)
- Carbaryl – We observed significant foliar burn in our field trials this year
- Imidan – Was not included in our trial but may have merit where aphids are not an issue
We are clearly in need of some new insecticide active ingredients for use in alfalfa. That is part of the rationale for the field trial we conducted this year. Hopefully, we will have some new insecticides to choose from in the future. The key with any insecticide program now and in the future is to use integrated pest management practices and to rotate insecticides to help avoid the evolution of insecticide resistance before it is too late.
- Author: Rob Wilson
Several experiments were conducted at IREC from 2011 to 2013 examining insecticides applied at planting for protecting onion stands from maggot damage. The results show seed treatments with OI100 (spinosad) or Sepresto (clothianidin + imidacloprid) were the most effective all three years. The next best labeled option was Lorsban applied in furrow at the maximum label rate. In untreated plots, onion stands were reduced at least 50% from maggot feeding compared to those treated with an insecticide.
During the study we trapped maggot flies to determine when adults emerge and deposit eggs in onion fields. The results from 2011, 2012, and 2013 are shown in the figure below. All three years, seedcorn maggot and onion maggot flies were found in the traps. Researchers in New York and Canada have developed degree day models to predict fly emergence based on air temperatures. Using Tulelake weather data, we compared their predicted emergence dates with fly counts from 2011-2013. Most years, the predicted emergence date came close to matching elevated fly counts.
Below is a graph showing predicted emergence of seedcorn and onion maggot using 2016 weather data. 2016 air temperatures and degree days in Tulelake from January to March 30th are similar to 2014. In 2016, peak first generation seedcorn maggot flight is predicted to peak in mid-April. Seedcorn maggot is widespread and most problematic in fields with a lot of decaying organic matter, especially following alfalfa. Onion maggot is also widespread and problematic in most onion fields. Onion maggot overwinter in onion cull material and flies are attracted to deposit eggs at the base of onions plants. Onion maggot is most problematic in fields located within 0.75 miles of old onion fields with cull material left in the field.
First generation seedcorn maggot larvae will be feeding from mid-April through May when onions are sensitive to maggot damage. Second generation seedcorn maggot fly emergence will likely occur early enough for seedcorn maggot to damage plantings in May.
- Author: Rob Wilson
- Contributor: Kevin Nicholson
- Contributor: Skyler Peterson
- Editor: Laurie Askew
Hooray for Snow! The wheellines are tied down, the mint still has been winterized, and snow covers the potato fields at the Intermountain Research and Extension Center.
The winter weather doesn't stop the work here, though.
Post-Harvest Potato Evaluations
Multiple research studies at IREC are investigating potato appearance and quality during storage. With Red and Yellow potatoes, washing and polishing is necessary to accurately evaluate new potato varieties appearance in the grocery store. This information assists potato farmers in selecting varieties that match consumer preferences.
IREC staff research associates Skyler Peterson and Kevin Nicholson are busy evaluating potatoes on more than 10 different characteristics including color, shape, and size. Potatoes even get inspected under the microscope to evaluate characteristics not visible by the naked eye. Consistency and accuracy are the focus of our staff.
All of this data is compiled and analyzed, and then published for use by farmers, industry personnel and other researchers. You, too, can learn more about potatoes by reading the Potato Reports on the IREC website.
