Alternaria dauci appears on leaves as small variously-sized dark brown to black lesions. The lesions often appear on the edges or margins of the carrot leaf. In severe cases, the lesions expand, causing the leaflets to turn brown, shrivel and die. The leaf may have a scorched appearance.
The petiole or leaf stems can also become infected and develop brown irregular-shaped lesions.
Generally, the older, lowest leaves of a carrot are affected before the upper younger leaves. The disease will first be obvious in carrot crops as irregular patches or 'hotspots' of diseased leaves.
Cercospora carotae appears as small, almost circular, brown spots that are often surrounded by a yellow border. Generally the upper, younger leaves are affected first. C. carotae is not as prevalent as Alternaria dauci.
In the past, outbreaks of Cercospora had symptoms that were indistinguishable from those caused by Alternaria.
Although symptoms of bacterial blight (Xanthomonas campetris pv. carotae) can be confused easily with those of alternaria leaf blight, the lesions of bacterial blight are smaller, with a characteristic yellow border. However, bacterial blight has only occasionally been observed in Western Australia and not in recent years.
Root scab complex or carrot scab may be caused by seed-borne Alternaria or severe blight outbreaks in the field. This disorder is characterised by thin corky black lesions arising on the secondary root nodes on the carrot. Fusarium species can usually be isolated from scab lesions on carrots taken from the field, but evidence suggests that the Fusarium may be a secondary invader.
Leaf blight spores are spread by water, wind and machinery. The spores may come from other diseased fields or from debris of decomposing carrot leaves.
Alternaria dauci can be introduced on infested carrot seed. Spores produced on infected plants are spread rapidly during wet windy weather.
- Control seed-borne Alternaria by treating seed with hot water (50°C) for 20 minutes, or soaking seed in a 0.2% suspension of thiram for 24 hours at 30°C.
- Carrot seed can be dusted with thiram (5g/kg of seed). Dusting is not as effective as soaking. Most seed companies take precautions against seed-borne disease. Check the seed tin label or with your seed company representatives.
- Rotary hoe the harvested leaves from old crops, as these contain fungal spores that can spread the disease to neighbouring, younger crops.
- Good crop nutrition will help limit leaf blight. Avoid excessive nitrogen fertiliser use, as very lush tops make blight difficult to control.
- Some carrot varieties are more tolerant of leaf blight than others. Stefano, for example, is highly tolerant of leaf blight while Mojo is susceptible.
- Fungicide sprays can play an important part in an integrated blight control program.
- Many of the currently registered fungicides act as protectants, not eradicants, by interfering with spore germination. Once the disease is present, these fungicides will not kill the fungus. Eradicant fungicides can be more effective once the disease is present however development of resistance by the fungus to the fungicide needs to be managed.
- The effectiveness of a spray program improves if the first spray is applied at the first sign of disease. Check lower leaves for signs of spotting.
- Field trials have shown that under high disease pressure, alternating weekly chlorothalonil and mancozeb gives excellent blight control. Cupric hydroxide can also be alternated with chlorothalonil. The spray program must begin at the first sign of disease, particularly in autumn when blight develops rapidly.
- Iprodione and vinclozolin failed to control leaf blight in trial work because these fungicides are ineffective against Cercospora.
- Some newer fungides may be effective in controlling leaf blight.
The registration and availability of chemicals for pest, disease and weed control changes regularly. Consult a trained and experienced horticultural agronomist or the Australian Pesticides and Veterinary Medicines Authority (APVMA) website for chemicals which are currently registered or have a permit for use on this crop.
The information on the label or permit for a chemical must be followed, including the directions for use, critical use comments, withholding period and maximum residue limit. Quality assurance (QA) schemes for horticultural crop production require producers to have current information on chemical registrations readily available.
Registered chemicals should be used in conjunction with integrated pest management practices as part of an integrated pest management program. Chemicals used are to comply with resistance management strategies.
This information is based on a DAFWA Farmnote originally prepared by Angie Galati and Allan McKay.