Latent Infection Diagnosis-based Strawberry Anthracnose Management
Anthracnose is one of the most destructive diseases of strawberry affecting fruit growers in the United States and all over the world where strawberries are grown. Although all parts (crowns, runners, petioles, blossoms, fruits) of the strawberry plant can be affected by the disease, the most obvious symptoms are expressed from crown and fruit infections.
Crown rot due to crown
infection manifests as wilting of the whole plant (Figure 1) and can result in
a total loss depending on the plant stage when infection takes place. Fruit can
be infected at the green or ripening stage, but the most conspicuous symptoms,
such as black sunken lesions (Figure 2), appear on ripening or fully ripened
Figure 1. Scattered plant wilting due to anthracnose crown rot in fruiting field.
Figure 2. Black sunken lesions on fruit indicating anthracnose fruit rot.
Anthracnose crown rot is a high
temperature-loving disease and is mostly a problem in the southeastern United
States where warm, moist conditions and abundant rainfall favor the growth and
rapid dissemination of the pathogen. However, anthracnose fruit rot can be a
problem anywhere strawberries are grown.
Causal Agent, Spread of the Disease and Diagnostics
Fungal pathogens belonging to the Colletotrichum gloeosporioides and C. acutatum species complexes cause ACR and AFR, respectively, with some minor exceptions. Although inoculum sources for plants in fruiting fields can be diverse, non-symptomatic infected (quiescent or latent) planting stock is one of the most important sources of inoculum. Thus, the use of disease-free transplants is the first and most effective method for controlling anthracnose in the fruit production field. Due to the vegetative propagation of strawberries, infection at any stage in the propagation nursery, plug production facility or fruiting field can pose a risk, although symptom expression on foliage is rare unless the infection level is very high. This infection biology poses a challenge to growers due to a lack of knowledge about the plant’s health and whether they would need to take any preventative measures to manage the disease in their fruiting fields.
Diagnostic evaluation of propagation materials for latent infection from representative samples can aid in decision-making for disease prevention and management. While fruit growers expect to secure disease-free plants, it is often difficult for suppliers to ensure truly healthy plants for the same reason mentioned above. Research on developing a sensitive and cost-effective latent infection diagnostic method is in progress.
Until a standard protocol for this purpose is in
place, plant health status based on available gramoxone protocol from nursery
producer to plug producer and eventually to fruit producer can help them making
appropriate management decisions. Laboratories at state land-grant universities
may be able to, for a fee, provide an assessment of plant infection based on a
pre-bloom herbicide dip of foliage. This can aid the fruit producers to decide
if an aggressive spray schedule will be needed or not. Plant suppliers can also
use this information to decide if plants are worth sending to fruit producers. The
herbicide dip method can be carried out in a laboratory by strictly following
safety health rules so that the herbicide does not get inhaled or come in contact
with the skin. Briefly, dip the sample as a group of 10 leaves in 70% ethanol
for two minutes (mix 700 milliliters of ethanol with 300 milliliters water),
followed by dipping in 0.525% NaOCl for two minutes (mix 30 milliliters bleach
in 970 milliliters water). Samples are then rinsed in sterile deionized water
for one minute, followed by holding leaves for a minute to remove extra water.
Samples are then dipped in 0.3% paraquat cations for one minute (mix 10
milliliters gramoxone in 990 milliliters water), followed by leaving the
samples on a sterile paper towel for two minutes to let the leaves dry up. Leaf
samples are then set the on mesh inside crispers that have layers of moist
paper towel. Samples are examined under the magnifying lens (at 15-times to
20-times magnification) for the presence of salmon- or peach-colored acervular
growth (Figure 3) after five to seven days. The availability of a fume hood
ensures the safety of workers.
Fig. 3. Upper side of a strawberry leaflet subjected to paraquat protocol showing acervular growth. (Note: Plant was inoculated with C. acutatum conidia three months prior to collection of samples.)
From our experience, if the pathogen is latently
present in a large proportion of plants at the pre-bloom stage, a fungicide
program starting from 10% bloom can save the crop from severe losses due to
AFR. A list of effective products can be found in the strawberry IPM guide:
https://smallfruits.org/files/2022/01/2022-Strawberry-IPM-Guide.pdf. The spray
interval should be dependent on the presence or absence of fungi on foliage and
prevailing weather conditions during fruit development and ripening.
Although strobilurins or QoI fungicides (FRAC
11) have been among the most effective against anthracnose, they are also
highly vulnerable to fungal populations developing resistance to them. These
products should only be used as directed in terms of consecutive sprays allowed
and the total amount used in a crop year. It is also very important to
communicate with plant suppliers about whether any of these products have been
used in the nursery or plug production cycle, to avoid excessive applications
of the same chemistry. Also, in a situation of control failure, the best option
is to rotate the fungicide mode of action. For example, if Pristine fails to
provide optimum control, Merivon should not be the alternative as both products
have active ingredients from the same FRAC, or Fungicide Resistance Action Committee,
group (7+11). In this case, Switch (FRAC 9+12) or a multi-site activity product,
like Captan or Thiram, should be considered as an alternative. All of these
products at the same time should provide good gray mold suppression.
In most cases, ACR symptoms showing up during fall or early spring suggest a need for a fungicide program. However, detection of the pathogen on transplants may indicate the need for a fungicide dip and subsequent initiation of a spray program within a month of planting. It is extremely important to determine whether crown rot and plant wilting are due to ACR or Phytophthora crown rot by sending samples to a diagnostic lab, as different products are used to manage these two diseases.
Author: Mahfuz Rahman, WVU Extension Specialist – Plant Pathology
Last Reviewed: April 2022
Trade or brand names used in this publication are for educational purposes only. The use of such product names does not imply endorsement by WVU Extension to the exclusion of other products that may be equally suitable.