Alternaria
Beginner
Scientific Name
Alternaria cucumerina
Identification
- Alternaria lesions first form on the leaves as small, yellow-to-brown flecks with a light yellow halo
- As the flecks enlarge in size they turn brown and necrotic; concentric rings also become apparent
- On muskmelons, alternaria lesions may grow as large as 2 cm (3/4 in.) in diameter; they are somewhat smaller on watermelons
- If left untreated, the lesions often grow together causing the entire leaf to turn brown, wither and die
- The resulting reduction in the leaf canopy often causes an increase in sunscald and damage to mature fruit
- Severely infected leaves become twisted and cupped
Often Confused With
Anthracnose
Scab
Septoria
Period of Activity
Alternaria develops under a wide range of environmental conditions. It can survive in temperatures from 12– 30˚C (54– 86˚F).
Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf. Alternaria is more commonly found in muskmelons and watermelons; however other cucurbits may occasionally become infected.
Thresholds
None established.
Advanced
Scientific Name
Alternaria cucumerina
Alternaria commonly infects both watermelon and muskmelon, however other cucurbits may occasionally become infected. Severe infections may cause premature defoliation, exposing the fruit to sunscald injury and reduced marketability.
Identification
Alternaria lesions normally form on the older leaves first. The initial infection causes small, yellow-to-brown flecks with a light yellow halo. As they enlarge, the flecks turn brown and necrotic. They also develop concentric rings.
In muskmelons, Alternaria lesions may grow as large as 2 cm (3/4 in.) in diameter. Watermelons lesions are a darker brown and somewhat smaller (5 mm or 1/4 in.). If left untreated, the lesions often grow together causing the entire leaf to turn brown, wither and die. The resulting reduction in the leaf canopy often causes an increase in sunscald and damage to mature fruit.
Often Confused With
Anthracnose
Scab
Septoria
Biology
Alternaria mycelium can survive for 1 to 2 years in cucurbit residue. Under humid weather conditions, dormant mycelium become active and produce conidia spores which then infect the current year’s crop. Secondary spores are spread to neighbouring plants by splashing water. They may also become airborne resulting in the further spread across the field.
Alternaria infections require periods of high humidity (2 to 24 hours) and can occur at almost any temperature from 12– 30˚C (54– 86˚F). The rate of infection increases at warmer temperatures. Heavy dews or intermittent rainfall followed by warm windy conditions favour the spread of this disease. Moisture promotes the production of conidia spores by the infected lesions. Windy weather then disperses the spores across the field and into neighbouring fields.
Period of Activity
Alternaria develops under a wide range of environmental conditions. It can survive in temperatures from 12- 30°C (54- 86°F).
Scouting Notes
Inspect 10 leaves at each of 20 random locations in the field. Observe both the older and newer leaf growth. Record the percentage of leaves infected and the average number of lesions per leaf. Alternaria is more commonly found in muskmelons and watermelons; other cucurbits may occasionally become infected.
Thresholds
There are no thresholds established for this disease. Fungicides applied at the first sign of symptoms will reduce the spread of this disease.
Management Notes
- Follow a 3- 4 year rotation away from all cucurbits.
- Initiate a preventative fungicide program at the first sign of disease or when the crop canopy closes over.
- Avoid heavy fertilizer applications.
- An unnecessarily dense, heavy canopy will prolong humidity on the leaf surface.
