Phomopsis obscurans

Phomopsis obscurans is the specific pathogen that is known to cause leaf blight of strawberries, and is among the list of most commonly found diseases caused by Phomopsis.[1]

The symptoms of leaf blight infections begin as one to several circular reddish-purple spots, 3/8” to 1/2” in diameter on a leaflet and enlarge to form V-shaped lesions with a dark brown inner zone, a light brown outer zone, and a purple, red, or yellow perimeter.[2] Lesions follow major veins progressing inward, which is what gives rise to the characteristic V-shaped lesions that result after the pathogen has established a strong infection in the plant tissue.[3] The whole leaflet may turn yellow and then brown due to infection and subsequent necrosis of the plant tissue. In severe cases where the pathogen has progressed substantially, the stolons, fruit, and petioles of the leaf become infected and may girdle and kill the stem. The disease is commonly found in young tissue of the strawberry plant, when defense mechanisms may not yet be fully established.[4][5]

In later stages of development, when the fruit is infected and begins to rot, the disease resembles that of Anthracnose fruit rot. However, the diseases can be quickly differentiated by their differing signs because the lesions of Anthracnose fruit rot do not develop the tell-tale pycnidia fruiting bodies found in Phomopsis.[4]

Temperature, inoculum density, moisture period, and host tissue maturity all have an effect on the disease development of the inoculated strawberry plant. Temperature takes a vital role in the infection and development of the blight disease caused by the pathogen, Phomopsis obscurans. The most ideal temperature that leads to the greatest amount of disease growth, conidial germination, and germ tube elongation is between 26 and 32 °C (78.8 and 89.6 °F). Within this temperature range, inoculated plants experienced the greatest symptom development and the most severe infection at 30 °C (86 °F).[3] These temperatures are commonly found in the midsummer environment of North Carolina, where leaf blight of strawberries caused by Phomopsis obscurans is especially prevalent.[3][2]

Inoculum density also has an effect on the development of the disease on host tissue. Inoculated conidial concentration is directly correlated with disease severity, thus, with increasing concentrations of conidia present on the host tissue, the severity of the disease also increases. The optimum disease severity was achieved with a concentration of 1x10⁷ conidia per millimeter.[3]

Similarly, duration of exposure to a moist environment also has an effect on disease severity. With the increase in time that inoculated plants were kept in a saturated moist environment, the disease severity also increased in a linear relationship. Ideal moisture exposure is 72 hours, which suggests that it takes the pathogen that long to establish optimal inoculation of the host tissue.[3] There are no insect vectors that were found to specifically influence the spread of this pathogen.

The age at which the host tissue is infected has an influence on the sustainability of the disease and subsequently its overall severity.[4] Young strawberry plants, around 25–30 days of age, have the highest susceptibility to the Phomopsis obscurans pathogen. Characteristics of young plant tissue, such as under-developed cuticles and cell walls, make the tissue highly susceptible to easy penetration of the pathogen and thus, more rapid colonization of the pathogen in the host tissue.[3]

There are no varieties with reported resistance to leaf blight; however, some varieties are more susceptible to the leaf blight pathogen than others. In the case of disease management, cultural practices are the most effective way of reducing the infection:

• Selection of Plant Material: Use certified disease-free plants to make sure that you are not introducing a different pathogen into your field and so your plants are not already faced with a reduced immunity before planting.[4]
• Sanitation: Be sure to clear the field of old plant debris from previous plants that have grown in the area. Overwintered fungus is commonly found in this plant debris, which can easily lead to a new fungal inoculum in the Spring when fresh plants are growing.[4]
• Site Selection: Plant your strawberries in a field with excellent soil drainage and good air circulation by allowing enough space between each plant. Because this pathogen favors wet environments,[5] any form of standing water will be a breeding ground for the pathogen and will lead to an outbreak of the disease. Also, be sure to plant the strawberries in a location with all-day sunlight and no shaded areas. Sunlight teamed with good air circulation will ensure that any excess water will evaporate and promote faster drying of plant parts after irrigation.[4][6]
• Irrigation Techniques: Irrigation methods are an easy way of controlling the amount of water that settles in the field. Applying overhead irrigation in the early morning to speed up the drying of leaves during the day is the most effective means of irrigation to prevent the growth of pathogens in this case. Overhead irrigation ensures that water isn't settling in the soil and contributing to a moist environment and delivering water to the plants from above ensures that any excess water not absorbed by the plants is evaporated by the sun throughout the day. It is also important to measure the correct amount of water that is being delivered to the field. This method of irrigation can be faulty if too much water is delivered to the plants and the excess settles in the soil and creates a moist environment anyway.[6]
• Control Weeds: Weeds commonly block the areas of air circulation between plants, which means water is more likely to settle on the leaves and not evaporate, thus favoring an environment for pathogen growth.[4]
• Fungicides: Spraying your strawberry plants with fungicides is a really effective way of controlling the plant pathogen because this disease is found on the foliage of the plant. Fungicides easily destroy the conidia and prevent further pathogen inoculations. However, fungicide use should not be the primary mode of control because it is expensive to repeatedly spray your plants with each new growing season. Captan and Thiram are common fungicides used on strawberry plants.[4][6]
• The bacteria, Pseudomonas fluorescens, is a proven biocontrol agent to control this fungal disease in an environment friendly way.

• Index Fungorum
• USDA ARS Fungal Database