What’s Cropping Up? Volume 30, No. 3 – May/June 2020

The full version of What’s Cropping Up? Volume 30 No. 3 is available as a downloadable PDF on issuu. This issue includes links to COVID-19 resources on the back page. And as always, individual articles are available below:

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Wheat spindle streak mosaic in spring: A reminder to plant a resistant wheat variety in fall

Gary C. Bergstrom, Plant Pathology and Plant-Microbe Biology Section, School of Integrative Plant Science, Cornell University

The Disease

Wheat spindle streak mosaic (WSSM), caused by wheat spindle streak mosaic virus (WSSMV), is a disease that attracts little attention today because most of our widely grown, winter wheat varieties have significant levels of resistance to it.  Yet WSSMV persists in New York soils in its protozoan vector ready to infect the roots of susceptible winter wheat varieties soon after planting.  Swimming spores (zoospores) of the vector move through films of water in the soil and thus root infection is favored by moist conditions in fall. Plants remain infected over the winter dormant period but do not develop typical leaf symptoms until spring following a number of weeks of cool weather which favors virus replication and virus movement from roots into shoots. Temperature, not moisture, is what drives symptom development in spring since plants were already infected in the fall. Only winter wheats, not spring wheats, are affected by WSSMV because of the time it takes to build up virus levels in the roots and then the shoots.

Symptoms

Symptoms of WSSM first appear in late April or early May and are characterized by long, light green, spindle-shaped streaks with dark centers (Fig. 1). As leaves age, these streaks can become necrotic and resemble lesions of Septoria tritici blotch but without dark fruiting bodies, i.e., pycnidia of Zymoseptoria, in evidence under a hand lens. Symptoms of WSSM fail to develop on new leaves that emerge when average daily temperatures exceed 60 F, though symptoms can reinitiate at later growth stages if persistent cool conditions occur during stem elongation, head emergence, and even grain-filling. Conditions have been ideal in April and May 2020 for development of WSSM.  Symptom development is extremely sensitive to warm temperatures such that we have seen very little WSSM in years with high temperatures in early spring.

wheat leaves
Figure 1. Characteristic symptoms of wheat spindle streak mosaic on wheat flag leaves at boot stage (A) and close-up of spindle streaks (B).

Management

What should a wheat producer do if she/he observes characteristic symptoms of WSSM this spring?  There is no action that can be taken to mitigate WSSM in a growing crop – the yield damage, which can exceed 30% of the crop’s potential, has already occurred. However, diagnosis of the disease is a sure reminder that the variety they are currently growing is susceptible to WSSMV, and they need to choose a variety with at least moderate resistance for planting in the coming fall.  This should elicit a conversation with your seed supplier about varieties resistant to WSSMV; some companies include that information on their website and in their seed catalogs but others do not.  While the majority of available varieties express resistance to WSSMV, susceptible varieties appear in the seed market from time to time. Scores for WSSM also are included in winter wheat variety trial tabular results (https://blogs.cornell.edu/varietytrials/small-grains-wheat-oats-barley-triticale/small-grains-cultivar-trial-results/) from Cornell’s Small Grains Breeding Program in years when symptoms are observed.

If you find more pronounced mosaic and fewer distinct streak symptoms in a variety designated to be WSSMV-resistant, your wheat could be infected by another soilborne, protozoan-transmitted virus called soilborne wheat mosaic virus (SBWMV), which we have diagnosed occasionally in isolated fields in southern areas of the Finger Lakes Region.  Resistance to SBWMV is independent from resistance to WSSMV, though it is also available from wheat seed suppliers in a choice of adapted varieties.

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What’s Cropping Up? Volume 30 No. 1 – January/February 2020 Now Available!

Soybean Cyst Nematode Now Confirmed in Six Additional Counties in New York

Jaime Cummings and Ken Wise, NYS Integrated Pest Management Program; Mike Hunter, Mike Stanyard, Aaron Gabriel and Kevin Ganoe, Cornell Cooperative Extension; Michael Dorgan, NYS Dept. of Agriculture and Markets

The soybean cyst nematode (SCN) is considered the number one pest of economic concern of soybeans nationally and globally, potentially causing 10-30% yield loss in the absence of above ground symptoms.  In 2017, national estimates reported over 109 million bushels lost to this pest in the U.S. alone.  Considering that this pest is confirmed in surrounding states and provinces, and given its potential to spread, statewide survey efforts have been underway since 2013 to determine the presence or absence of the soybean cyst nematode in NY.  From 2013-2016, numerous fields in 17 counties were sampled and tested as part of a statewide soybean disease survey led by Cornell’s Field Crops Pathology program, funded by Northern NY Agricultural Development Program and NY Corn and Soybean Growers Association.  In 2016, SCN was confirmed in one field in Cayuga County by Cornell’s USDA ARS Nematology lab, albeit at very low levels.  Since then, survey efforts have continued, because it is widely assumed that SCN is much more prevalent in NY.

In 2019, the NYS Integrated Pest Management Program was commissioned by NYS Department of Ag and Markets to coordinate a Cooperative Agricultural Pest Survey (CAPS) in soybeans with Cornell Cooperative Extension specialists to maintain vigilance against potentially invasive species.  For more information about the CAPS program and this survey effort, please refer to this article.  As part of this survey, 25 soil samples were collected from fields in 16 counties across NYS and were submitted for testing at the SCN Diagnostics laboratory.  Of those 25 samples, seven of them were positive for SCN in six different counties, confirming our suspicions that this pest is potentially widespread throughout soybean production areas in NY.  This brings us to a total of seven counties in NY with at least one field positive for SCN.  The counties identified with fields positive in 2019 include Columbia, Dutchess, Jefferson, Monroe, Tompkins and Wayne (Fig. 1).

SCN NYS location by county
Figure 1. Soybean cyst nematode survey efforts in NY since 2013. Counties colored in green had fields tested with negative results, and counties colored in red had one or more fields that tested positive. The first positive result was in Cayuga County in 216. In 2019, six more counties tested positive as a result of the soybean Cooperative Agricultural Pest Survey.

Thankfully, the egg counts in these positive samples were all below 500 eggs per cup of soil (250 cc of soil).  Although that may sound like a lot, these are very low numbers compared to the 10,000-80,000 egg counts that some growers struggle with in other states.  This means that we are in a good position to proactively manage for this pest before it gets out of hand and starts causing economic losses.

An integrated management approach will help NY soybean growers stay ahead of the soybean cyst nematode.  This involves continued testing efforts to monitor your fields for SCN.  Determining if you have the pest is the first step toward management.  For detailed information and recommendations on how to collect samples for SCN testing and where to send those samples to, please refer to this article.  If you get a positive result, keep records of your egg counts for each individual field.  Implement the following tactics when managing for this pest:

  1. SCN can be moved among fields on soil, whether it be via wind, water, equipment, or boots. Consider improving sanitation of equipment coming from fields with known SCN infestation to avoid spreading it to others.
  2. Crop rotation is the number one tool for managing SCN. Rotating to a non-host crop, such as corn, small grains, alfalfa, forage grasses and mixes for one year can reduce the nematode population by up to 50%.  Continuous soybean production in an infested field can increase nematode populations exponentially, since this pest can have up to three life cycles per season in NY.
  3. Select and plant soybean varieties with resistance to SCN, and rotate those resistant varieties that you plant.  The nematode quickly develops resistance to the resistant varieties when exposed to the same varieties over and over, in the same way that weeds develop resistance to over-used herbicides.
  4. Consider nematicidal seed treatments if your SCN populations start causing economic damage (Fig. 2). Research has shown that these products are only cost-effective with high SCN population levels causing significant damage.
  5. Keep testing. Continue to test fields that you get negative results from, and especially continue to test fields that you get positive results from.  Keep track of your egg counts in each field to know how your populations are changing, as that may affect your management strategy.  It is recommended that as long as egg counts remain below 30,000 eggs per cup of soil, crop rotation with SCN-resistant soybean varieties is the best approach.
SCN seed treatment table
Figure 2. Nematicidal seed treatments available for managing soybean cyst nematode.

Crop rotation is the most important tool, and we are lucky to have a number of non-host crops already in our rotations.  But, SCN has a fairly wide host range, including a number of our common weeds and cover crops.  Some of these weed and cover crop hosts include chickweed, some clovers, common mullein, henbit, pokeweed, vetch and purslane (Table 1).  That’s just another thing to remember as you plan your crop rotations and weed management strategies.

SCN host plants table

Keep in mind that testing for SCN can be tricky, since it can be difficult to detect at low population densities, and populations can be quite variable within a field (Fig. 3).  Focus your testing efforts on fields with unexplained lower yields, or fields with a history of Sudden Death Syndrome (SDS) or Brown Stem Rot.  It is well known that there is a strong correlation between the presence of SCN and SDS.  If you see patches of SDS in your field, that would be an ideal location to pull soil samples for testing for SCN.  For more information on the relationship between SDS and SCN, please refer to this article.

Variability of SCN within field
Figure 3. Grid sampling reveals high variability in soybean cyst nematode population densities within a single field. (Image courtesy of Iowa State University)

For more information on this pest and recommendations, please visit the Soybean Cyst Nematode Coalition website.  There you will find numerous resources explaining the resistance issues with this pest, how and where to test for it, management recommendations, and success stories.  Expanded SCN testing efforts will commence in 2020, supported by the NY Corn and Soybean Growers Association.  If you suspect SCN in your fields, contact your area Cornell Cooperative Extension specialist for assistance, they may be able to offer you free testing on suspect fields as part of the expanded testing efforts in 2020.

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Soybean Cooperative Agricultural Pest Survey: Vigilance against Potentially Invasive Species

Jaime Cummings and Ken Wise (NYS Integrated Pest Management Program), Mike Hunter, Mike Stanyard, Aaron Gabriel and Kevin Ganoe (Cornell Cooperative Extension), and Michael Dorgan (NYSDAM)

Cooperative Agriculture Pest Survey Header
 Image courtesy of Purdue University CAPS website

Annual funding in the Plant Protection Act 7721 supports the Cooperative Agricultural Pest Survey (CAPS) pest detection program, led by the USDA Animal and Plant Health Inspection Service (APHIS), to safeguard against introductions of potentially harmful plant pests and diseases.  These surveys ensure the early detection of potentially invasive species that could negatively impact U.S. agriculture and/or environmental resources.  The NYS Department of Agriculture and Markets (NYSDAM) works with APHIS to prioritize the potentially invasive species to monitor in economically important commodities in NY each year.  In 2019, NYSDAM partnered with the NYS Integrated Pest Management (IPM) program to coordinate a soybean CAPS survey to monitor for two potentially invasive moth species, as well as to expand monitoring of the soybean cyst nematode across New York soybean production areas.

The overarching goal of the CAPS program is to monitor for species that shouldn’t be here, and to confirm that they still aren’t in NY or even the U.S.  These surveys are often the result of cooperation among state and federal employees, such as APHIS pest inspectors, NYSDAM inspectors and extension specialists.  This ‘boots on the ground’ approach allows for broad coverage of the surveys across the state involving many individuals with agricultural and pest identification expertise.

Larva and moth
Figure 1. Golden twin spot moth and looper larva. (photos by S. Hatch and P. Hampson, Bugwood.org)

For the 219 soybean CAPS survey, two moth species that are already problematic elsewhere in the world, but not known to exist in the U.S. were selected.  The Golden Twin Spot moth (Chrysodeixis chalcites), which currently causes yield losses in Africa, Europe, the Middle East and Canada, has a larval stage known as a ‘looper’ which can cause significant damage to soybeans, tomato, cotton, tobacco, beans and potatoes (Fig. 1).  Feeding by the loopers can result in defoliation, and they can also cause foliar damage due to rolling leaves with webbing for nests.  The Silver Y moth (Autographa gamma), which is already a concern in many countries in Asia, Europe and Africa, also has a caterpillar larval state that can cause significant damage to soybeans and many other agronomically important crops, including beets, cabbage, hemp, peppers, sunflower, tomato, potato, wheat, corn and wheat (and many more) (Fig. 2).  These caterpillars also defoliate and harm leaves through rolling and webbing.  Given how potentially damaging an introduction of these pests could be to U.S. agriculture, it’s important that we are vigilant in our efforts to monitor for them and ensure they aren’t in NY.

Silver Y moth and larva
Figure 2. Silver Y moth and caterpillar larva. (photos by P. Mazzei and J. Brambila, Bugwood.org)

In addition to monitoring for these two moth species, we also prioritized a pest that has very high potential to affect soybean yields in NY, and one that has thus far only been confirmed in one field in NYS.  The soybean cyst nematode (SCN) is considered the number one pest of soybeans nationally and globally, causing an estimated 109 million bushels of yield loss in the U.S. in 2017.  Extensive collaborative sampling for this pest from 2014-2017, supported by the NY Corn and Soybean Growers Association and Northern NY Agricultural Development Program, was coordinated by Cornell University and Cornell Cooperative Extension programs.  Over the four years of the SCN survey, numerous fields in 17 counties were sampled, and one field in Cayuga County was identified as positive for SCN in 2016, albeit at very low levels (Fig. 3).  Though it’s promising that SCN wasn’t identified widely across NY, we are fairly confident that it is very likely in many more than just one field in one county.  Given the potential impact this pest could have on NY soybean (and dry bean) production, we decided to include this pest in the 2019 CAPS survey.

Soybean Cyst Nematode
Figure 3. Soybean cyst nematode survey efforts in 17 counties in NY from 2014-2017, with one positive ID in Cayuga County in 2016, and information from the SCN Coalition on why you should test for SCN.

Six collaborators (Jaime Cummings and Ken Wise of NYS IPM, and Mike Stanyard, Mike Hunter, Aaron Gabriel and Kevin Ganoe of CCE) spent part of their typical summer soybean scouting efforts from western, to central, to eastern and northern New York setting up and checking pheromone traps intended to monitor for the Golden Twin Spot moth and Silver Y moth (Fig. 4).  They communicated the importance of these surveys to cooperating farmers who agreed to host these traps in 25 fields across the state.  Any suspicious moths caught in the traps are submitted to the Cornell Insect Diagnostic Clinic for thorough identification.  Thus far, we have not caught any Silver Y or Golden Twin Spot moths.  And that’s good news!  As the growing season winds down, we will collect soil samples from the same 25 fields for SCN testing at the SCN Diagnostics laboratory.

CAPS survey distribution
Figure 4. Distribution of the 2019 soybean CAPS survey.

A funding proposal to continue this work in 2020 has been submitted.  If accepted, it may also be expanded to include a corn CAPS survey for other potentially invasive pests with additional locations in southwest and central NY.  For more information on the national CAPS program, please visit their website.  For additional information on the soybean cyst nematode, please visit the SCN Coalition website, and check out these resources on SCN efforts in NY:  Soybean Cyst Nematode Now Confirmed in NY, Sudden Death Syndrome and Soybean Cyst Nematode in Soybeans, Fall is the Time to Test for Soybean Cyst Nematode.

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Managing Oat Crown Rust to Prevent Yield Loss

Michael R. Fulchera, Gary C. Bergstroma, Mark E. Sorrellsb, and David Benscherb
aPlant Pathology and Plant-Microbe Biology Section and bPlant Breeding and Genetics Section, School of Integrative Plant Science, Cornell University, Ithaca, NY

Crown rust is a continuing threat to oat production in New York, and recent epidemics have cast a spotlight on this disease. To better advise growers on crown rust management, we examined the impact of crown rust on oat grain yields and the disease resistance of available and soon to be available varieties.

The fungal pathogen that causes this disease, Puccinia coronata var. avenae, is widespread in New York and often found on susceptible oat varieties. Characterized by bright-orange, blistering pustules, this disease can be seen from June through August (Figure 1). Once established in a field, disease progresses quickly as the spores of the fungus are dispersed by the wind. The spores are blown to new leaves, different plants and even other fields. Older crown rust lesions develop a black rust spore stage, and these spores can infect the alternate host, common buckthorn, providing early inoculum for oat infections in fields adjacent to infected buckthorn in the following May.

Pustules of crown rust
Figure 1. Orange-brown uredinial pustules (bearing urediniospores) of crown rust on oat leaves.

The pathogen requires living plants to survive so it rarely persists through the winter on oat in New York. However, viable crown rust spores from maturing oat crops in states to our south arrive in New York on wind currents each spring to commence annual epidemics. Some overwintering can occur in New York when the fungus moves back and forth between oat and common buckthorn (Figure 2).

Aecia of crown rust
Figure 2. Yellow-orange aecia (bearing aeciospores) of crown rust on buckthorn leaves in May.

Management of crown rust is best achieved through careful selection of an oat variety. Few options exist to combat the disease after plants are in the field. Some fungicides are labelled for crown rust control in New York, and some growers have realized a return in investment from a timely fungicide spray at or prior to panicle emergence. Crown rust significantly impacts the yield of susceptible varieties and in extreme cases may cause crop failure. Even slight visual symptoms around the soft dough growth stage can translate to yield loss (Figure 3). Rust pathogens are known to evolve quickly to overcome resistance, but based on several years of observation we have identified the varieties that currently are most resistant in New York (Table 1). If you are considering a spring oat planting, choose a variety with proven resistance to current populations of the crown rust fungus in New York.

Bar chart showing effect of crown rust on oat yields
Figure 3. Effect of crown rust infection on oat yields.
Crown rust infection can significantly impact spring oat yields. This plot shows the average predicted yields observed at different disease severities. This data was taken from 360 small research plots spread across western, central and eastern New York in 2015-17. The amount of crown rust damage to flag leaves in each plot was measured during early grain filling. Even when visual disease severity recorded at the soft dough growth stage appears as low as 5%, yield may be limited by crown rust.

Crown rust susceptibility tableLate summer forage plantings are at a higher risk for infection since the spores that cause disease will increase and spread throughout the growing season. When these forage plantings are infected, pathogen overwintering on buckthorn can be increased. This contributes to crop epidemics the following year and may speed the breakdown of oat varietal resistance.

Crown rust will continue to threaten oat yields, but you can reduce the spread of this disease by planting resistant varieties and notifying your local Cornell Cooperative Extension Field Crop Specialist or the Cornell Field Crops Pathology Program if you find the pathogen in your fields.

Reference on the complete study:

Fulcher, M.R., D. Benscher, M.E. Sorrells, and G.C. Bergstrom. 2020. Preserving spring oat yields in New York through varietal resistance to crown rust.Plant Health Progress Published On-line 21 January 2020 https://apsjournals.apsnet.org/doi/10.1094/PHP-05-19-0037-RS

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