Carrots are grown year round in California by having production in various locations in the state. The southern San Joaquin Valley is the most concentrated area of carrot production, but other areas of production include the southern desert valleys of Imperial and Coachella, the high desert Antelope Valley and Cuyama Valley near the Central Coast. The diverse growing areas allow year round production of carrots for the nation's two largest carrot growers and shippers that are located in Kern County.
Fall is when a major portion of the carrot acreage is ready for harvest in Kern County. Typically a variety of foliar diseases can be found on the fall carrots. The warm fall days and occasional late fall storms can be ideal conditions for foliar diseases to develop. This is particularly true with Alternaria leaf blight (ALB), which can be especially troublesome in some years. This fall is probably one of those years with ALB being found easily in many fields.
Leaf blights can be caused by several different plant pathogens and can be difficult to distinguish from each other. The symptoms may look very similar but can be caused by completely different microbes. Often it is necessary to have the help of a trained person to differentiate the cause of leaf blights. Properly identifying the disease is very important because it determines which treatments are best to use.
The most damaging foliar disease of California grown carrots is Alternaria leaf blight. Although there are effective control methods, it continues to cause considerable losses in some fields. Alternaria leaf blight (Alternaria dauci) can cause considerable defoliation. Defoliation reduces yields indirectly due to less green leaf tissue for plant growth. More importantly however, yields are reduced because the carrots cannot be lifted by the tops for mechanical harvest due to the weakened tops. Many carrots are left behind during harvest when the tops are damaged by Alternaria leaf blight.
Individual lesions of ALB appear as dark-brown to black necrotic lesions along the margins of the leaf blade. Lesions on the petioles are more distinctive with a tan lesion with a black border or are entirely black. Symptoms appear first on the older foliage and become more severe as the leaves mature. The lesions can become so numerous and severe that the older leaves defoliate leaving only the small young leaves.
Lesions of early blight of carrot (Cerocospora carotae) are circular tan lesions and may be located on any part of the leaf while Alternaria leaf blight is more commonly found on the leaf margins. Early blight also tends to be more of a problem in the coastal areas but can be occasionally found in the interior valleys in years with cool, wet autumns.
Bacterial leaf blight (Xanthomonas campestris) is another leaf blight of carrots that can be difficult to distinguish. While Alternaria and Cerocospora are both fungi, Xanthomonas is a one-celled bacterium. Bacterial leaf blight can be very difficult distinguished from ALB. The lesions on the leaves and petioles of bacterial leaf blight are black and appear very similar to lesions caused by Alternaria leaf blight. However bacterial leaf blight lesions tend to be watery or greasy in appearance when they first appear. Often times the lesions will have a slight yellow halo. Examining lesions on the petiole is the best way to distinguish between alternaria leaf blight and bacterial leaf blight. The lesions of ALB will be tan with a black outline or entirely black. The lesions of bacterial leaf blight will be brown slightly water soaked in appearance. Also they may exhibit a shiny ooze that is varnish-like in appearance.
Alternaria and bacterial leaf blight can both be carried on the seed. Using clean seed prevents introducing these pathogens to a field. Most seed companies will provide the disease index for individual seed lots. Hot water soaks along with fungicides have proven to be effective treatments to ensure that the seed is clean and are used by most carrot seed companies today.
These microorganisms may also survive in the soil on previous infected carrot debris. However they cannot survive in the soil on their own. Once the debris is broken down these plant pathogens will die. After harvest the remaining crop debris needs to thoroughly incorporated into the soil so that it may decompose. Rotating out of carrots for two to three years will assure that no surviving inoculum remains in the soil.
Fungicide treatments may be required for Alternaria leaf blight and early blight when spores are blown into a field from volunteer carrots or other nearby fields. This is true more so in the fall when environmental conditions are often ideal for the disease to occur. An early application of a fungicide at the 3 to 4 leaf stage is commonly used to keep disease pressure low. Frequent scouting of the fields will help determine when and if any other applications are needed.
Bacterial leaf blight is best controlled with copper fungicides. Other types of fungicides will not control bacterial leaf blight. Copper fungicides are sometimes used as a preventive for Alternaria leaf blight and Cercospora leaf blight, but once disease is detected then chlorothalonil, iprodione, or other types of fungicides are needed. Again, properly identifying the pathogen is important in determining which treatment is best to use.