Association of Pythium and Phytophthora with Pre-emergence Seedling Damping-off of Soybean Grown in a FieldConverted from a Paddy Field in Japan

Abstract

In Japan, soybean is usually cultivated in fields that have been converted from rice paddies, and poor seedling establishment due to pre-emergence seedling damping-off is often observed during the rainy season. In this study, the factors that cause the damping-off in flooded soil were investigated under high soil moisture conditions in a greenhouse and in agricultural fields. In sterilized soil sampled from a soybean field, seedlings emerged well under 48-hr flooded conditions. In unsterilized soil, soybean seeds treated with the fungicide, mancozeb +metalaxyl exhibited much higher emergence rates even under 48-hr flooded conditions than the seeds treated with oxytetracycline +streptomycin, benomyl, or flutolanil. Pythium, Phytophthora, Mucorales, Trichoderma, Geotrichum-like microorganisms, and some fungi producing conidia in a false head, were isolated from decayed seedlings. Of the isolated microorganisms, oomycete microorganisms, Pythium helicoides, other Pythium sp., and Phytophthora sp. were pathogenic to soybeans under flooded conditions. As the length of the flooding period increased, pre-emergence seedling rot also increased. However, the pathogenic oomycetes rarely caused pre-emergence seedling rot under conditions without flooding. Furthermore, under flooded conditions, the damage caused by these pathogens was reduced by treating the seeds with mancozeb+metalaxyl. These results indicate that oomycete microorganisms are involved in pre-emergence seedling damping-off under flooded soil conditions.

Does Pre-Germination Flooding-Tolerant Soybean Cultivar Germinate Better under Hypoxia Conditions?

“…When seeds are physically injured, microbial infection can occur more easily, with infection by microbiological organisms being partly responsible for pre-emergence seedling rot under flooded conditions (Kato et al, 2013). In 5 cultivars, excluding 'Peking', the number of seedlings that did not produce root hairs and/or lateral roots increased under hypoxia and higher temperatures (Fig.…”

Variation in the Resistance of Japanese Soybean Cultivars to Phytophthora Root and Stem Rot during the Early Plant Growth Stages and the Effects of a Fungicide Seed Treatment

“…Because the infection by zoospores is essential to initiate the parasitic phase of the P. sojae life cycle, fields saturated for a long time provide one of the most ideal conditions for inducing Phytophthora root and stem rot. In Japan, most soybean plants are cultivated in fields converted from rice paddies ( Kato et al, 2013 ; Sugimoto et al, 2012 ). Such soybean fields usually contain poorly drained soil with a high moisture level, which is ideal for a P. sojae infection and subsequent multiplication.…”

Improvement of Soybean Seedling Establishment under a Flooded Condition by Seed Coating with Molybdenum Compounds

“…Kokuryu et al (2010) reported that the inhibition of the seedling establishment was attributed to the imbibition damage, soil microorganisms and soil surface strength. Kato et al (2013) reported that microorganisms especially oomycetes are involved in the flooding damage. Accordingly, these problems need to be solved to reduce the flooding damage of soybean seedlings.…”

“…However, the result did not deny the possibility that the improvement might also be caused by a mechanism other than the inhibition of the generation of sulfide ions. For example, the molybdenum compound might improve the seedling establishment by directly inhibiting the multiplication of microorganisms like the microbicide, because Kato et al (2013) reported that a microbicide improved the establishment of soybean seedlings under a flooded condition. Nakayama et al (2005) pointed out a difference in the flooding damage of soybean before and after radicle protrusion.…”

“…Nakayama et al (2005) pointed out a difference in the flooding damage of soybean before and after radicle protrusion. The flooding damage of soybean before radicle protrusion seems to be mostly caused by the physical disruption through rapid water absorbance, the decomposition due to microorganism and so on (Nakayama et al, 2005;Kokuryu et al, 2010;Kato et al, 2013). The flooding damage of soybean after radicle protrusion seems to be mostly caused by oxygen deficiency, hardness of soil surface and so on (Nakayama et al, 2005;Kokuryu et al, 2010).…”