Anthracnose has been diagnosed this week in two commercial plantings in North Carolina. This is always bad news and destroys early optimism that this disease was not going to be a problem this year. For growers who have had anthracnose diagnosed there are several things that should be done as soon as possible:
Due to hail damage and other freeze/wind damage effects, we have had several cases of problems that look like anthracnose but is not. It is damaged tissue colonized by Alternaria fungi, a secondary invader. Anthracnose lesions tend to be distinctly circular during initial infections and have an orange tinge. Alternaria lesions can be irregular in shape and have a distinct greenish tinge. There is no concern if Alternaria fruit spot occurs. It is a secondary problem.
Phytophthora crown rot is widespread this year within the state. Three years ago we had a few cases of the problem and there was a distinct link to plant source. However, last year, especially after Hurricane Floyd, and this year, the problem appears more widespread and no single plant source can be identified as the problem. Why are we seeing more Phytophthora crown rot and what does it mean for our industry? The spread of the pathogen, Phytophthora cactorum, is probably distributed in a similar fashion as several of our other disease problems such as anthracnose and angular leaf spot. It can be associated with the plants that are distributed from one geographic location to another. However, unlike anthracnose and angular leaf spot, once the pathogen is introduced in a field, there is a high likelihood it can persist in the soil for years. This pathogen is a cousin to the pathogen that causes black shank in tobacco, a disease in which many in North Carolina are familiar. It is a water mold and therefore is spread and favored by free-standing water. This pathogen can form structures called oospores that can persist in the soil for years. These oospores can germinate when a host is present and one of the life stages, zoospores, can swim in the water and encyst on strawberry roots. The pathogen then grows along the root and directly into the crown where it can cause considerable crown discoloration. If damage is not enough to stunt or kill the plant initially (in the fall or early spring) plants will suddenly collapse during hot weather and under a heavy fruit load because of extensive damage to the water conducting tissue between the roots and shoots.
What to Do?
I was very aggressive in working with growers last fall and spring to devise management strategies to limit plant losses. However, many growers are seeing problems now for the first time. Historically, the problem has been restricted to a few scattered plants within each acre. Regretfully, I am observing fields with a higher incidence. Some growers are still electing to apply Ridomil Gold EC through the drip line to limit plant losses. According to the label, an application during harvest is allowed (always follow the label for timing, rates and application methods). Ridomil is systemic and will be absorbed into the plant and carried in the water conducting tissues. It is curative if a plant is not heavily infected and if the strawberry can produce sufficient adventitious roots to support the crop load and transpiration water requirements. Because the fruit do not have stomata, they do not transpire and therefore a limited amount of water and Ridomil enters the fruit. The chemical accumulates in the roots, crown and ultimately the leaves. Many infected plants will continue to collapse after a Ridomil Gold application due to the extent of damage already incurred.
Employment and program opportunities are offered to all people regardless of race, color, national origin, sex, age or disability. North Carolina State University, North Carolina A&T State University, U.S. Department of Agriculture, and local governments cooperating.
Last modified on May 15, 2001 by Stephen J. Toth, Jr.
