ALTERNARIA LEAF AND STEM SPOT, ALTERNARIA BLIGHT

Causal organism:
Alternaria alternata, Alternaria helianthi, Alternaria zinniae

Description:

The dark brown spores of Alternaria alternata are borne in simple or branched chains from the tips of simple dark conidiophores and are divided into several cells by transverse and vertical walls. Mature conidia typically 10-30 x 5-12 µm, short conical beak or beakless, narrowly ellipsoid to ovoid and elongated on branching chains, dull olive in colour, 3-7 transepta, 1-5 longisepta, individual chains of 5-15 conidia, complex of branching chains may contain up to 50-60 conidia.

Conidia of Alternaria helianthi are solitary, nonbeaked, and borne on simple or (rarely) branched conidiophores. Conidia are cylindrical to elongated - elliptic, straight or slightly curved, yellowish brown in colour, septate, with 4 to 11 transverse or longitudinal septa, constricted in the septa and rounded at both ends. The size of conidia is 100.4 µm in length and 25.1 µm in width.Alternaria zinniae produces brown conidiophores, septate, erect to slightly bent simple, mostly solitary, measure 38-88.2 x 4-9 µm. Conidia cylindrical double walled, solitary, olivaceous-brown, scarred at base 5 - 9 cross and 2 - 4 longitudinal septa, simple, straight to slightly curved and pointed beak. The conidia are 117.6 - 243 m in length and 10.5 - 25.2 µm in width.

Host range:
Safflower and cocklebur can also be alternate hosts of A. helianthi.

Occurrence and importance:
A. helianthi is recognized as a major disease in more humid areas in central Europe, India, Australia, South America and parts of Africa. In these areas, yield losses may range from 15 to 90 %, with oil losses from 20 to 30 %. Alternaria is reported to cause losses from 50-60 % in more humid areas.
Two species of Alternaria cause leaf and stem spots on sunflower Alternaria helianthi A. zinniae, of which A. helianthi is the more prevalent and more serious. These diseases can be serious in warm humid environments. A. alternata, a common saprophytic species, is often associated with declining plants, but its significance is unknown. Yield losses may occur through reduced head diameters, number of seeds per head, and oil content or quality.

Symptoms:

Both A. helianthi and A. zinniae produce dark brown and striated spots on leaves. These spots are irregular in size and shape with a very dark border and a grey centre. The spots on young plants may have a yellow halo. Leaf lesions may coalesce causing leaves to wither. Stem lesions begin as dark flecks, which enlarge to form long, narrow lesions.

 

. These stem lesions often coalesce to form large blackened areas resulting in stem breakage. Stem lesions are randomly distributed on the stem and are not associated with the point of attachment of the leaf petiole.

 

 

 

 

Dark brown oval to circular spots with a target board appearance can form on heads. If disease is severe, plants may be defoliated prematurely and die or frequently lodge.

 

 

Disease cycle:
The mycelium rests in the plant debris covering the soil and less frequently on seeds. The fungi can be seed-borne at low levels although seed may be relatively unimportant as a source of the inoculum under most conditions. Seedling blights caused by Alternaria may develop when sunflower plants emerge in rainy weather on Alternaria-infested land. However, plants at the flowering to maturing stage are more susceptible than plants in the vegetative or budding stage. Conidia are spread by the wind and by the water. Splashing conidia affect the lower leaves as the first. The leaf spots resulting from this primary infection will sporulate and produce new conidia, which are then spread to the whole foliage and will contaminate new plants causing a general epidemic on that parcel. High temperature (24-27°C) and frequent rain (free water or dew present for a few hours) alternating with periods of dryness is favourable to the sporulation of the fungus.

Control:
Disease is managed through practices that such as crop rotation, destruction of the culture debris and tillage operations that bury and rapidly promote residue decomposition. Early planted fields are generally more susceptible to severe disease losses than later planted ones. Plants are most susceptible at flowering and during seed fill. Seed treatment with Captan fungicide significantly reduces the incidence of Alternaria seedling blight. Foliar fungicides with active substances of benomyl (Fundazol), vinclozolin, (Ronilan) and iprodion (Rovral) can be used for control alternaria blight.

 
Commercial name
Active compounds
Group of pesticides
Rate
Action mode
Efficicacy
Fundazol 50 WP benomyl benzimidazols 1 kg.ha-1 systemic White mould, Phomopsis grey spot, Grey mould, Alternaria leaf spot
Rovral Flo iprodione dikarboximids 2-3 1.ha-1 systemic
contact
White mould, Phomopsis grey spot, Grey mould, Alternaria leaf spot
Sportak Alpha prochloraz + carbendazim mix 1,5 l.ha-1 local systemic White mould, Phomopsis grey spot, Grey mould, Alternaria leaf spot
Sumilex 50 WP procymidone dikarboximids 1,5 kg.ha-1 contact White mould, Grey mould, Alternaria leaf spot
Rovral 50 WP iprodione dikarboximids 1,5 kg.ha-1 contact White mould, Grey mould, Alternaria leaf spot