|Colletotrichum crown rot (anthracnose crown rot) caused by Colletotrichum gloeosporioides and C. fragariae.|
Dan Legard, University of Florida, Gulf Coast Research and Education Center, Dover, FL 33527
Updated October, 18, 2000
Colletotrichum was first reported infecting strawberry in Florida by Brooks in 1931. He reported a disease of stolons and petioles that was caused by Colletotrichum fragariae. Subsequently he also reported that it caused a wilt and crown rot in 1935.
In the 1960ís it was found that C. gloeosporioides could also cause a crown rot and wilt of strawberry, and this species has since become the predominant cause of Colletotrichum crown rot in Florida. Occasionally a third species of Colletotrichum, C. acutatum, has been found to cause a slow decline and ultimately a crown rot of strawberry. However, the development of symptoms with C. acutatum is not typical of the rapid plant wilt and death caused by C. gloeosporioides and C. fragariae.
Colletotrichum crown rot can be a serious disease in Florida and the Southeastern United States. The disease cause severe losses, killing up to 80% of the plants when susceptible cultivars are used. This disease can also be important in other subtropical production regions where warm temperatures and frequent rainfall can occur during the season.
Strawberry transplants appear to be the most likely source of primary inoculum of C. gloeosporioides for crown rot epidemics in Florida. Because strawberries are clonally propagated, the pathogen can easily be transported from an infested nursery to a fruiting
field on strawberry transplants. Often, crown rot epidemics on different farms are associated with plant material from the same nursery. Weed and other non-cultivated host plants of C. gloeosporioides may also provide inoculum for epidemics of crown rot. Some isolates of C. gloeosporioides from plants around strawberry fields are capable of causing crown rot when inoculated into strawberry crowns, however, comparison of DNA markers between isolates from weeds and those from strawberry suggest that weed isolates are not associated with disease outbreaks in Florida (Non-cultivated hosts are not important sources of inoculum for colletotrichum crown rot on strawberry in Florida.). It is possible that inoculum from weeds and other hosts are important in nursery production areas, and are the source of primary inoculum that infests nursery production fields and ultimately contribute to epidemics in Florida, but more research needs to be done to investigate this possibility.
Because the spores (conidia) of C. gloeosporioides and C. fragariae are produced in sticky masses, they are primarily dispersed
by water, or by adhering to harvesters, farm equipment and insects. Although the sexual stage for C. gloeosporioides has not been observed in the field, RAPD marker analysis of C. gloeosporioides isolated from crown rot diseased plants within fields in Florida has found that populations of the pathogen are highly polymorphic, and an analysis of these data suggest that this variation is indicative of a sexually mating population (Oversummer survival in Florida of Colletotrichum gloeosporioides in strawberry crowns.). These results suggest that within field movement of conidia (asexual stage) during the season may not be important for spread of Colletotrichum crown rot disease in annual production systems. Since conidia of these pathogens are not wind dispersed, their intermediate and long distance movement is primarily restricted to activities associated with transplanting (i.e. movement on diseased or infested plants) and harvesting.
Over-wintering (or over-summer in winter production regions) of inoculum in soil or on plant debris is another potentially important source of inoculum. This is especially important in Florida, because the production fields are replanted with strawberry every year. Recent research in Florida, has found that C. gloeosporioides can survive in strawberry crowns, but that it disappears from the crowns as they break down during the summer (Oversummer survival in Florida of Colletotrichum gloeosporioides in strawberry crowns.). Under certain conditions it may be possible for the pathogen to survive for several months in plant debris, even in plant debris from apparently health strawberry plants. Because of the potential for the pathogen to survive between seasons in plant debris, it is important to kill strawberry plants after the end of the season and incorporate the crop debris into the soil in a timely fashion to speed its breakdown and limit the survival of C. gloeosporioides.
Ultimately the control of Colletotrichum crown rot of strawberry requires a combination of chemical and cultural methods. The most effective methods for controlling this disease are preventative in nature; because once the disease becomes apparent in the field it is often too late to control it. Preventing the introduction of the pathogen into the production field is critical for effectively controlling this disease. This could be accomplished by using certified pathogen-free transplants. Unfortunately, currently available plants are only certify plants as being free of viruses. Often times these plants are also certified free of certain diseases, but the methods used in meeting certification guidelines do not insure that the plants are pathogen-free, only that they are disease-free. In the absence of pathogen-free transplants, the best way to control the disease is by using resistant cultivars. Although no strawberry cultivars are completely resistant to C. gloeosporioides, there are substantial differences in susceptibility to crown rot among cultivars. Cultivar Sweet Charlie is moderately resistant to crown rot, while the cultivars Camarosa, Oso Grande, and Festival, are highly susceptible to Colletotrichum crown rot.
The effectiveness of fungicides for controlling Colletotrichum crown rot in fruiting fields is unclear. It may be that the incubation time between infection and disease is so long that most fungicides are ineffective in controlling the disease. However, regular fungicide applications in the nursery may be effective in controlling the spread of the disease in transplants. Most labeled fungicides like Captan and Thiram are not systemic and would not provide much control in the fruiting field. The systemic fungicides Benlate (benomyl) and Topsin M (thiophanate-methyl) can both provide excellent control of C. gloeosporioides if the isolate population of the pathogen is not resistant, and treatment of fruiting fields with these products can be very beneficial. Unfortunately, isolates of C. gloeosporioides resistant to these fungicides are commonly found infecting strawberry.
Copyright © 2000 [Legard]. All rights reserved