Black Leg Disease (Phoma lingam)
The disease has four main stages on winter oilseed rape:
1. The most important sources of infection for newly-emerged plants are air-borne spores produced on oilseed rape stubble after harvest. Fruiting bodies that produce air-borne spores need about 20 days with rain to mature, eg spores were released early after the wet August in 2005 and in 2006, but late in 2003 when that month was dry.
2. Air-borne spores, released mainly on rainy days, infect leaves to produce the leaf spot stage. Symptoms appear after 5-7 days at 15-20°C but take over 30 days to develop at 3°C.
3. No symptoms are visible while the fungus grows from the leaf spot, down the petiole, to the stem. The growth rate down the petiole may be up to 5mm/day at 15-20°C but slows to 1mm/day at 3-5°C. Fungicides give no control once the stem has been infected.
4. The fungus spreads within the stem, leading to visible stem canker symptoms about six months after leaf infection. Early leaf spotting leads to early stem cankers, which are most likely to reduce yield potential.
Weather: August and September rainfall is the key factor determining the onset of leaf spotting. Above average rainfall, particularly in August, indicates early risk.
Leptosphaeria maculans or Phoma lingam survives the intercrop period as mycelium and pseudothecia in crop residues. In Canada, leaf tissue does not persist long enough to allow pseudothecia to develop, but pseudothecia do form on basal stem tissue. Upon maturity, the pseudothecia produce ascospores.
Ascospores of the fungus are released after rainfall when temperatures are between 8-12ºC/46-54ºF. These spores can be wind-dispersed for hundreds of meters (yards). Pycnidia can and do overwinter readily in stubble, but because pycnidiospores are not airborne to any significant extent, they are of minor importance in initiating the first cycle of disease.
Ascospores germinate in the presence of free water from 4-28ºC (40-82ºF). Penetration is through stomates. The pathogen also may be seedborne. Seeds may be infected and/or infested by the pathogen. Infected seeds can give rise to infected seedlings, but levels of seed contamination are always very low. Primary infections usually occur on the cotyledons or basal rosette leaves of the plant. Wet weather favors these primary infections.
The fungus invades the intercellular spaces between the palisade and epidermal layers of the leaf. This symptomless biotrophic phase is followed by invasion of the mesophyll with the resultant death of cells and the appearance of gray-green lesions. The hyphae continue to ramify through the leaf tissue until they reach a leaf vein. The fungus then colonizes the cortex and/or xylem parenchyma of the petiole. At the junction of the petiole and the stem, the fungus invades the stem cortex where it causes a canker. It is at this point that stem resistance is expressed and determines the ability of the disease to proceed to the damaging stem canker phase. Stem cankers form when the plants bolt (produce an upright stem from the rosette on which the flowers form). Stem cankers develop most quickly at 20-24ºC/68-75ºF and are most severe under stress conditions such as mechanical, insect, or herbicide injury.
Pycnidiospores (conidia) are released from the pycnidia under moist conditions in a mucilage, a watery, sticky solution. These spores are responsible for secondary cycles of the disease, but ascospores are the more important source of inoculum because they are more infective and are airborne.
The pynidiospores are dispersed to new infection sites by rain splash. Pycnidiospores germinate more slowly than ascospores and require more than 16 hr of continuous wetness at the optimal temperature range of 20-25ºC/68-77ºF. The minimum latent period (the time from infection to the production of new inoculum) following infection by pycnidiospores is 13 days. Although secondary infections by pycnidiospores do occur, most losses are due to primary infections of leaves by ascospores that lead to basal stem cankers and eventual lodging of the plants.
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