Last week, samples of frogeye leaf spot of soybean were brought into the lab. On the underside of the characteristic lesion were the conidia. This came from an area where the incidence of frogeye was notable at the end of the season. For the 2018 season a susceptible variety was planted back into that same field. Environmental conditions have been favorable for this disease to begin in some areas of the state.
However, most of the varieties in the state have very good levels of resistance to this disease and if good rotation is practiced it will take more time for enough inoculum to build up to begin to move around the state.
There are numerous fungicides available for this foliar pathogen and in Ohio, but the caution here is the the strobilurins. Based on sampling over the past 3 years, the samples of this fungus in 2017 (last year) indicated that most of the populations were now resistant to the strobilurin based fungicides.
We will begin testing any samples received this year in earnest, but based on last year’s sampling, farmers who are managing this disease should focus on using a triazole (FRAC Group 3) or a MBC Thiophanate (FRAC Group 1, thiophanate methyl) at the higher rates if disease is active in the field. The best timing is one spray at R3, at the end of flowering.
This fungus will only infect young newly expanding leaves, so the goal in the spraying is to protect those big flushes of leaves as our indeterminant soybeans really fill out. Fungicide coverage should focus on the upper third of the canopy for this disease.
Other soybean diseases to keep in mind.
1. Early and mid-season Phytophthora. With each of these saturating rains, soybeans that have low levels of partial resistance, (tolerance) will continue to develop Phytophthora stem rot. Last week at North West Branch the susceptible variety was dying all over the field.
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At this point, the plant should be able to hold its own against this pathogen if the resistance package is there. Make notes if it is not and choose a variety with better resistance scores for 2019.
White mold caused by Sclerotinia stem rot. Cool nights and random rains to keep the moisture levels up are perfect for this disease of soybean. However, only in historically infested fields and only if the canopy is closed at those first flowers and most often on highly susceptible cultivars.
So double check your variety ratings that got planted in those areas with a history of white mold. Based on a very large study by my team, resistant varieties did not need the fungicide and the fungicides that have worked were boscalid and picoxystrobin. Do not use other strobilurins in these fields as those have enhanced disease development. One application at first flower or right before (1 to 2 days) provided the best management of this disease on highly susceptible varieties.