Phoma Leaf spot

Leptosphaeria maculans

 In England Phoma is the main serious autumn disease problem for oilseed rape and later sown crops are at greatest risk. The pathogen's impact is more noticeable in the following summer when stem canker develops and plants ripen prematurely or lodge where stems are weakened. Typical yield losses in unprotected crops of susceptible varieties are in the range of 0.5 to 0.7 t/ha.

How to spot Phoma
Phoma leaf spot infection can begin from September as crops emerge. The initial symptoms are white to fawn circular lesions wh
These large leaf lesions are caused by Leptosphaeria maculans (Phoma A). A second species, Leptosphaeria biglobosa (Phoma B) produces small dark lesions with few fruiting bodies. L. biglobosa can spread to stems but has less impact than L. maculans on yield. Subsequently the fungus grows from the leaf spot to the stem via the petiole. The rate of this growth varies widely; from 5mm per day at 20°C down to 1mm per day at 5°C. In summer these stem cankers cause lodging and premature ripening.ich become dotted with small black fruiting bodies. These leaf lesions are green underneath. Occasionally they cause partial leaf death before winter, but generally have a minimal effect on growth until spring.
Sunken brown stem canker symptoms appear around six months after initial infection. They gradually enlarge, girdle the stem and weaken it leading to early ripening, lodging and plant death. There may be some spread to pods which develop brown lesions with a black margin, potentially leading to premature ripening and seed infection which can be a secondary source of infection spreading to new crops. Although a second type of spore (pycnidiospore) is produced on leaf lesions, these are considered to be of minor importance under UK conditions.

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