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Seed decay in Prairie soybeans

Phomopsis seed decay is the number one soybean problem in Ontario. 
Now it’s this disease-causing fungi may be coming to a field near you

Phomopsis seed decay is the No. 1 problem in soybeans in Ontario, says Ontario Ministry of Agriculture and Food plant pathologist Albert Tenuta. “The Prairie provinces are also starting to see some phomopsis issues as well,” he says. “Anywhere in the northern climate where you’ve got some delay of harvest, particularly under cooler weather conditions or wetter conditions, you’ll end up seeing increased potential risk for phomopsis. Whether it’s Ontario, Quebec or the Prairie provinces, the disease knows no boundaries.”

Phomopsis seed decay is caused by fungi in the genus Diaporthe. The same fungus also causes pod and stem rot. Where the fungus exists there is potential for both diseases, says Tenuta. Phomopsis decay can seriously impact seed quality, yield, viability and vigour, so knowing how to identify it and what to do once you do is critical.

Phomopsis is easy to distinguish from other soybean diseases, says Tenuta. The fungal disease produces a white or gray mould that is crusty in appearance. Unlike similar soybean diseases, Phomopsis will not appear “fluffy” and cannot be rubbed off easily with the fingers. The plaque-like mould will create fine wrinkles and cracks in the seeds, making them appear shrivelled and more lightweight. “Seeds may also have black specks (pysnidia) of the fungus present,” says Tenuta. “Seedlings resulting from infected seed may have small, reddish-brown lesions or streaks on the cotyledons or lower stems.”

The fungi that cause phomopsis seed decay (and pod and stem blight) overwinter in soybean residue, but seeds may also be infested, serving as the source of primary inoculum. For this reason, it’s important to choose your seed carefully.

Soybeans, says Tenuta, are infected early in the season, although visible symptoms will not appear until the later reproductive stages. Pod infection occurs between growth stages R5 and R6. Growth stage R5 is the beginning seed stage, where seed is about three mm long in the pod at one of the top four nodes on the main stem. By growth stage R6, green seed fills that same pod.

Seed infection occurs near the R7 growth stage, when one pod on the main stem has reached its mature colour. The infection causes pod colonization to decline dramatically because seed moisture drops. Seed infection, says Tenuta, will not take place once moisture is below 19 per cent.

“The key is harvesting in a timely fashion,” says Tenuta.

Early maturing varieties will often see higher incidence of phomopsis seed decay simply because they tend to mature during periods when weather is favourable for the fungus. Losses really depend on the season.

“Some years we could see five to 10 per cent of soybeans in a certain region affected,” says Tenuta. “There are some areas in Ontario — for instance, in the Niagara region — where in some years you could see 20 per cent of the fields or higher with incidence of Phomopsis.”

“Most fields have some degree of phomopsis in them,” he continues. “It’s just whether or not you go from lower to higher incidences, which is often driven by the weather. In some cases, growers may not even notice it under more mild conditions because the kernels may be blown out of the combine. They’re lighter weight, shrivelled.”

Phomopsis in Manitoba

Field crop pathologist Holly Derksen with Manitoba Agriculture, Food and Rural Initiatives says that since soybeans are relatively new to Manitoba, Manitoban soybean growers are still be in the “honey-moon” phase when it comes to disease issues.

“I, personally, do not have any experience with phomopsis seed decay as it has not been an issue in the recent past,” says Derksen. “I’ve been told that we have seen it in Manitoba in years where there is a late harvest with wet conditions and it can provide problems with germination. However, there are seed treatments available that are quite effective.”

She’s right. Usually, seed treatments will increase germination and emergence of soybean seed. Tenuta says, however, that distorted seed with visible fungal growth will still often fail to germinate — even when treated.

“Remember, no seed treatment will turn a lousy seed sample into an excellent sample,” he warns. “The impact on seed quality can be significant. Therefore, the use of good quality, clean seed will reduce early season stand problems.”

Where phomopsis is present, seed disease management strategies are critical. In areas with a history of the disease, plant pathogen-free seed or resistant varieties. Tillage and crop rotation can also reduce the amount of inoculum available to infect the crop. Finally, always scout soybeans and harvest soybeans destined for export or seed.

“Growers should be doing a pre-harvest scout of their fields. Just as plants are getting into the later stages of reproduction, late stages of development, before all the leaves are gone, they should — while they’re still green — take a look at them and look for any of these diseases,” concludes Tenuta.

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