Phialophora gregata

Phialophora gregata is a Deuteromycete fungus that is a plant pathogen which causes the disease commonly known as 'brown stem rot of soybean. P. gregata does not produce survival structures, but has the ability to overwinter as mycelium in decaying soybean residue. Two strains of the fungus exist; genotype A causes both foliar and stem symptoms, while genotype B causes only stem symptoms. Common leaf symptoms are browning, chlorosis, and necrosis Foliar symptoms which are often seen with genotype A are chlorosis, defoliation, and wilting. There are many ways to manage Phialophora gregata. The most effective form of management is disease resistance, but crop rotation, tiliage, SCN management, and changing the pH of the soil can also be effective Phialophora gregata’s infection of a soybean plant is accompanied by browning of the plant’s vascular and pith tissues. The plant often exhibits chlorosis and necrosis, as well as leaf browning. Wilting and defoliation are also known to occur.Signs of infection often go unnoticed until reproductive stages of a plant’s life cycle. They can be diagnosed earlier on by opening the stem and visualizing the pathogen. One can visualize signs by cutting open the stem in early stages of infection, but symptoms do not become apparent until after the soybean pod formation. Depending on which strain infects the plant, and what the environmental conditions are, the effect is more or less potent. Genotype A causes browning of stems as well as foliar symptoms such as interveinal chlorosis, defoliation and wilting. Symptomatic leaves have a shriveled appearance, but remain attached to the stem. Genotype B causes only browning of stems. Secondary symptoms of brown stem rot are stunting, premature death, decrease in seed number, reduced pod set, and decrease in seed size. Disease from P. gregata is easily confused with Fusarium wilt, due to the similar vascular symptoms observed in both. The diseases could be differentiated through growth on isolation media. The two diseases can be further distinguished by splitting the stems. A split stem with Fusarium infection would have tan or light brown discoloration in the cortex and a normal white pith, while a split stem with P. gregata would have a discolored, reddish brown pith. Root rot and blue masses of spores are symptoms only caused by Fusarium. As the disease is soilborne, it is not uncommon to find clusters of diseased plants together. The pathogen proliferates in stem tissues when soil has high moisture content and air temperatures from 70-80 degrees Fahrenheit. Rainy seasons promote brown stem rot. When soil is untilled, the disease is more likely to persist in soil. The incidence of brown stem rot is highest at soil pH ~6.3.