Insect-Viral Complex

BY Dairyland Seed Agronomy Team

As a follow up to last weeks soybean aphid article, we are still receiving calls and comments about oddities being observed in relatively new growth even as we begin to see the soybean plant start to naturally senesce. Many of these oddities are notable and can often be confused with or complicated by other disease presence. Viral infections seldom cause significant yield reduction unless pest pressures are extreme or complexed with multiple pathogens. As last season’s very mild winter followed by this season’s varied mix of moisture and temperature created conducive avenues for multiple insects to vector a pathogen(s) to the plant. While not all inclusive, below are several insect vector complex pests to be aware of in the Dairyland Seed footprint. The insects that most commonly transmit viruses in soybean are the bean leaf beetle (Cerotoma trifurcata), the soybean aphid (Aphis glycines) which we covered last week, as well as, other migratory aphid species, and soybean thrips (Sericothrips variabilis). Plants can be infected by multiple viruses when insect populations and suitable host environments create positive scenarios. Recognizing the unique plant-insect-virus relationship is the key to effective management.

Bean Pod Mottle Virus (BPMV) and Bean Leaf Beetle

BPMV is primarily carried by the bean leaf beetle throughout its lifecycle with symptoms tending be most evident in cooler temps. Leaf distortion, with raised wrinkling, and a mottled appearance is common symptomology of newer growth of infected plants. Infection can suppress pod development and may induce some bleeding from the hylum. Consider planting insecticide-treated seed if overwintering survival of bean leaf beetles is predicted, or if bean pod mottle has been confirmed in fields previously and bean leaf beetles have been present. Foliar-applied insecticides can manage bean leaf beetle populations and may reduce bean pod mottle.

Alfalfa Mosaic Virus (AMV) and 15 different aphid species including Soybean Aphid covered last week

Bright yellow and dark green tissue on older leaves as well as new growth is common. Newest growth may be stunted with this same yellow-brown discoloration. Many alternate hosts exist besides soybean include alfalfa, many other leguminous crops, and solanaceous crops. Can be far more prevalent in field edges and when boarding alfalfa.

Soybean Dwarf Virus (SbDV) and Aphids

SbDV is transmitted by aphids and has multiple legume and non-legume host plants. Leaves can be dark green, brittle, wrinkled, curled, small, and thick while green stem, delayed maturity, plant stunting, yellowing between leaf veins, and decreased seed number can also occur. Severity and incidence of soybean dwarf can increase if early season environmental conditions favor aphid development. Soybean fields planted next to clover are at higher risk of exposure to SbDV inoculum, while soybean seeds are not a source of inoculum.

Soybean Vein Necrosis Virus (SVNV) and Thrips

SVNV is a Tospovirus and can cause damage to multiple crops including soybean. To date, as many as 14 thrips species are known to be vectors of tospoviruses. Soybean thrips (Neohydatothrips variabilis) have recently been confirmed as a vector of SVNV and additional investigation may reveal other vector species.

Symptoms in the canopy appear randomly or clustered in distribution. Chlorotic yellow tissue along leaf vein gives way to red-brown irregularly shaped lesions where typically the dead and dying tissue falls away. Also, confusion with herbicide damage from ALS inhibitor and ACCase inhibitor herbicides is not uncommon.

Many times, these viral invader symptoms may often be confused with many common pathogens as symptomologies can be quite similar. The many fungal and bacterial pathogen’s that pose potential confusion risk are listed below. Look-alikes make solid ID and quantification of losses difficult and complicated for viral invaders. See associated links from the Crop Protection Network for more information.

Septoria Brown Spot https://cropprotectionnetwork.org/encyclopedia/septoria-brown-spot-of-soybean

Bacterial Blight https://cropprotectionnetwork.org/encyclopedia/bacterial-blight-of-soybean

Bacterial Pustule https://cropprotectionnetwork.org/encyclopedia/bacterial-pustule-of-soybean

Cercospora Leaf Blight https://cropprotectionnetwork.org/encyclopedia/cercospora-leaf-blight-of-soybean

Downy Mildew https://cropprotectionnetwork.org/encyclopedia/downy-mildew-of-soybean

Phyllotactic Leaf Spot https://cropprotectionnetwork.org/encyclopedia/phyllosticta-leaf-spot-of-soybean

Sudden Death Syndrome https://cropprotectionnetwork.org/encyclopedia/sudden-death-syndrome-of-soybean