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A second species of gall midge associated with widespread white mold in Minnesota soybean fields: Factors that favored both

James Kurle, Plant pathologist; Bob Koch, Extension entomologist; Dean Malvick, Extension plant pathologist; and Bruce Potter, Extension IPM specialist

Part 2. The fungus. Why was white mold so prevalent in 2019?

Figure 1. White mold mycelium with sclerotia on senescing
soybean stem. Photo: R. Koch
White mold (Sclerotinia sclerotiorum) has been unusually severe and widespread this year because of ideal environmental conditions, inoculum produced in previous years, and the prevalence of susceptible soybean varieties.

Environmental requirements for white mold

Moderate temperatures and high relative humidity, ideal conditions for disease development, have accompanied all soybean reproductive stages.

Sclerotia are specialized, dark, hardened, masses of mycelia (Figure 1) that allow the white mold fungus to persist in fields for several years. Abundant precipitation in July and August stimulated germination of soilborne sclerotia to form spore producing fruiting structures (apothecia).

Favorable environmental conditions, including moist soil and moderate temperatures, have resulted in the continual formation of apothecia throughout the growing season. As a result, apothecia have been available whenever flower petals are present, particularly when canopy formation and flowering coincided.

The apothecia eject ascospores that are capable of infecting the soybean plant whenever flower petals are present. Infection of soybean begins with the germination of ascospores that land on flower petals that act as a source of nutrients for the hyphae and developing mycelium.

Ascospore germination leads to disease development when mycelial infection attacks plant tissues. Moderate temperatures and high relative humidity promote this stage of infection. If relative humidity is high enough (>60%) the fungus can be visible on the outside of stems as white cottony “tufts” of mycelium (Figure 1). White mold symptoms become obvious with the appearance of leaf and stem necrosis and dead plants. The gall midge Karshomyia caulicola is associated with, and presumed to feed on, white mold mycelia.

Susceptible soybean varieties

This year’s white mold outbreak seems to be especially severe, including some varieties with relatively good white mold resistance ratings. Several Roundup Ready 2 Xtend® varieties appear to be heavily affected.

There are a couple possible explanations. The gene conferring dicamba tolerance may be associated with increased susceptibility to white mold. This should be investigated. However, there is another possible explanation – the background genetics of these varieties may be susceptible to white mold. With the release of the first Roundup Ready® varieties in 1996 and 1997, there were outbreaks of white mold similar to those we are seeing with the “Xtend” system. The Roundup Ready gene may have increased susceptibility, resulting directly or indirectly in increased risk of white mold.

Two things had happened. Growers planted high populations in narrow rows because cultivation was no longer necessary, and thus created ideal conditions for white mold. In addition, it was revealed that disease resistance was not a primary consideration when many of the first Roundup Ready® varieties were released. We have also seen severe brown stem rot and probably stem canker this year, further suggesting that disease resistance was lacking in some of the varieties growers selected to plant this year.

How accurate are ratings for white mold susceptibility in soybean varieties? Ratings for resistance to white mold in soybean varieties are problematic at best. The best resistance evaluation would take place in the field under intense disease pressure with ascospore inoculation accompanied by prolonged irrigation. We know of only two publications where direct ascospore inoculation was used for resistance evaluation. Their results are very different from results using other plant inoculation methods.

 Some seed company descriptions state variety susceptibilities as “moderately tolerant”, “average tolerance”, “little tolerance”. What does any of that mean? It is not much to go on, especially when the method used to obtain ratings is unexplained. Ratings were probably obtained in years with “some” disease pressure, but nothing like the situation that has developed in some areas of Minnesota during 2019. Furthermore, resistance ratings are not standardized among seed companies. Resistance is a product of multiple factors including resistance to infection by ascospores in petals, progress of infection in the stem, stem infection, and oxalic acid produced by the fungus. Resistance is also influenced by plant architecture (open, upright canopy, lodging resistance, and lanceolate leaves).

High inoculum levels 

In recent years, some areas of Minnesota have experienced severe white mold, producing large amounts of persistent sclerotia. Additionally, many broadleaf weeds, including pigweeds and ragweed, are susceptible to the fungus. During 2019, high inoculum levels and numerous days of disease-favoring weather challenged many varieties, not just those known to be highly susceptible. Populations of the gall midge, Karshomyia caulicola, appear to have increased along with those of its fungal food source.
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