By Dr. Thomas T. Yamashita
Dwarf disease of strawberries, also known as ‘crimp’ in some regions, is a disease caused by infestations of leaf nematodes of the genus Aphelenchoides. This disease is sometimes differentiated into ‘spring dwarf’ and ‘summer dwarf.’ The major differences between these two diseases are the species of nematodes responsible, and the times of year when they occur.
Spring dwarf is caused by Aphelenchoides fragariae and A. ritzemabosi, while the culprit for summer dwarf is A. besseyi. Spring dwarf affects strawberry plants early in the growing season, with symptoms becoming evident in April or so, while summer dwarf tends to make its presence known during hotter summer months.
Despite these differences, the symptoms of the diseases and methods of control are largely the same, and thus we can treat this as a singular disease.
Long forgotten in some regions of the United States, strawberry crimp has seen a resurgence in recent years.
Strawberry dwarf disease has impacted American agriculture since the 19th century. But the disease has been underappreciated for many decades. Until recently, the most comprehensive and readily available description of the disease, its causes, and means of control was an article published by the U.S. Department of Agriculture publication in 1943.
The leaf nematodes which cause strawberry dwarf can survive and often thrive in more than 250 plant species ranging from rice to violets. This has allowed these pests to maintain a toehold in areas where generations of growers have gone unaware of the threat in their midst. But now that the disease is impacting strawberry crops in regions where it hasn’t been seen for decades, understanding strawberry dwarf has newfound relevance.
Strawberry dwarf disease can readily identified by the characteristic crimped leaves and disfigured fruit that is symptomatic of infection.
The most obvious early symptom of strawberry dwarf is, appropriately enough, stunted growth. Young strawberry plants will be significantly diminished in size, with leaves developing a rough texture and looking crimped, curled, or twisted. In some cases, the edges of leaves may turn red before becoming brittle and turning brown.
Infected plants sometimes never reach full size. However, in many cases plants will experience late accelerated growth, ultimately reaching larger than average size. However, infected plants produce fewer flowers than normal and develop fruit late. When mature, the fruit is gnarled and cat-faced in appearance, sometimes described as looking like broccoli.
The leaf nematodes which cause crimp in strawberries can transported in numerous ways, which has contributed to the disease’s notable reappearance.
Wind and water are a common, passive means of transporting leaf nematodes from plant to plant. Irrigation is a particularly effective means of distributing nematodes, as they require the presence of moisture on plant surfaces in order to move.
The introduction of infected plants into existing fields is also a common means of transport. A nationwide outbreak of strawberry dwarf disease in 2016 was traced back to a single nursery in North Carolina.
In addition, leaf nematodes have been known to attach themselves to bees and hitch a ride until their transport lands on a strawberry plant or other suitable host.
Once a leaf nematode encounters a plant, it may feed on the surface for a while, or immediately burrow into the tissues. Nematodes tend to congregate and feed at the bases of young leaves, as well as in leaf folds.
The nematodes can stimulate increased vegetative growth in order to increase the supply of food, which explains the telltale accelerated plant growth late in the season. This manipulation of growth processes comes at the cost of flower and fruit development, resulting in the reduced number of flowers and disfigured, often unsellable fruit.
Dwarf disease can be easily diagnosed, but controlling the disease can be challenging.
Diagnosing dwarf disease is a simple matter of taking a sample of plant tissue and visually examining it under a microscope. Population density varies depending on the time of year, but can often exceed 150 nematodes per gram of leaf tissue.
It’s critical to identify the presence of the disease as soon as possible, before nematodes can propagate throughout affected fields. Speedy diagnose is important because treatment options are limited. Many of the popular nematicides and fumigants used in decades past are no longer legal to use in the United States. While it’s worth trying the nematicide products currently on the market, there’s little data as to their effectiveness in killing the leaf nematode species responsible for strawberry dwarf disease.
We have seen some success in soaking infected seedlings in hot water measuring 115 to 120 degrees Fahrenheit for 10 minutes, and then cooling the plants with cold water. Other experts in the field have found that potted plants immersed in 120-degree water for 20 minutes will resume normal growth with a complete cessation of symptoms. However, this treatment is not practical for many growers.
The most effective means of controlling the disease is by removing infected plant material from fields. Current evidence indicates that the leaf nematodes in question cannot survive in soil without plant debris to feed on for more than a few months. On the other hand, researchers have observed that if nematodes of the species A. ritzemabosi are allowed to reside on drying plant matter, they will enter a state of suspended animation in which they can survive for years. Thus, removing infected plants and plant matter is critical in returning fields to a nematode-free state.
Reducing the use of overhead irrigation technique can help to reduce the spread nematodes via water splashing across leaves. In addition, increasing spacing between plants can make it more difficult for nematodes to spread between plants.