Soybean seeds are generally infected by Phomopsis longicolla T. W. Hobbs, the principal cause of Phomopsis seed decay (PSD) and the most aggressive and endemic seed pathogen in soybean production areas worldwide.

Periods of high humidity, frequent precipitation, and warm temperatures during pod development favor latent infection of pods by P. longicolla. When these conditions persist during seed development and maturation, pod infection can lead to seed infection and decay.

Seed infection is even more pronounced when harvest is delayed beyond harvest maturity (defined as the first time the seed dries to <14 percent moisture).

Harvesting at or immediately following harvest maturity will lessen the risk of PSD and allow producers to harvest seed with good quality and greater yield. Periods of uninterrupted rain, however, may prevent harvest at or immediately following maturity resulting in increased levels of PSD and poor seed quality.

Seed severely infected by P. longicolla are shriveled, elongated, cracked, and often have a white, chalky appearance with a significant loss in germination. Seed with no visible symptoms of PSD might still be infected and have reduced germination and vigor.

The incidence of PSD infection by P. longicolla directly impacts seed quality by increasing the number of moldy beans, lowering test weight, and increasing the number of split beans. Hence, P. longicolla negatively impacts the value of infected seed for future plantings by reducing germination and vigor, and negatively impacts market value of the seed for grain when it is discounted or even rejected at the elevator.

Control of PSD with deep tillage, crop rotation, fungicide use and weed management has been inconsistent and ineffective. Application of appropriate fungicides at R3 and R5 growth stages may only be effective in improving seed quality if field conditions during harvest are conducive for harvest equipment to enter the field. The most effective strategy to combat this disease is through the deployment of PSD-resistant cultivars.

Multiple investigators have reported that resistance to PSD exists among some plant introductions (PI) and some breeding lines. Evaluation and selection of most of these soybean accessions for resistance against Phomopsis/Diaporthe has been commonly performed under rain-fed (no overhead-irrigation) environments in fields with harvests occurring at harvest maturity or immediately thereafter.

As a result, soybean accessions were not subjected to the environmental conditions that might have enhanced PSD. Studies have demonstrated that overhead irrigation in August and September resulted in an increase in seed infection by Phomopsis spp. Typically, PSD was enhanced by delayed harvest when warm temperature, free moisture, and high humidity prevailed on a continuous basis after harvest.

The selection of P. longicolla-resistant soybean accessions may be better achieved by using a screening protocol that includes overhead irrigation and delayed harvest.

However, the identification of P. longicolla-resistant accessions has been limited because of the lack of a consistent classification scheme across experiments and years. It is difficult to compare genotypic differences across environments, years, and research programs without a standardized system for assessing susceptibility.

A group of researchers, from USDA, the American Chestnut Foundation, and the University of Missouri, have attempted to measure the combined effects of irrigation and time of harvesting on seed infection caused by P. longicolla and to illustrate a method that may best evaluate resistance under the above environments.

The researchers found that the percent seed infection by P. longicolla was greater when accessions were irrigated during the growing season and harvested at normal maturity. Some lines such as MO/PSD-0259 were rated as resistant to P. longicolla (7 percent seed infection) when not irrigated and harvested at maturity, but were rated as susceptible (39 percent seed infection) with irrigation and delayed harvest.

The researchers state that the results indicate that simple comparisons of percent seed infection among accessions without irrigation, without delayed harvest, and without a standard control of a similar maturity group might result in false conclusions about resistance to P. longicolla.