Root colonization by Phymatotrichum omnivorum and symptom expression of Phymatotrichum root rot in cotton in relation to planting date, soil temperature and soil water potential

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Abstract
Cotton was grown in containers of soil with sclerotia of Phymatotrichum omnivorum. Plants were grown for 3 weeks in the greenhouse and then transferred to field plots. Foliar and stem symptoms of Phymatotrichum root rot occurred 15–50 days after field placement. Mean incubation periods (defined as time from field placement to symptoms) were about 24 days for placements made in June and July and about 35 days for placements made in August and September 1984. Colonization of roots was more advanced in the top 24 cm of soil. The time taken for symptoms to appear was not related to the amount of root colonization at the time plants were transferred to the field. In the later two planting dates a drop in soil water potential to approximately −25 bars after 20–24 days was associated with the delay in symptom expression. There were significant linear relationships between the length of the incubation period (days), or its inverse (day−1), and the independent variables: mean maximum daily temperature (°C) in the top 12 cm layer of soil, soil water potential in the 31–60 cm layer of soil and their product (representing the interaction of these two variables). In general, symptom expression was faster at higher temperatures and with wetter conditions. Newly‐formed sclerotia were found most frequently in containers some 5–6 weeks after plant death when soil temperatures had decreased and soils were close to saturation (September and October).