Ascochyta Diseases Of Pea

Ascochyta blights occur throughout the world and can be of significant economic importance. Three fungi contribute to the ascochyta blight disease complex of pea (''Pisum sativum''). ''Ascochyta pinodes'' (sexual stage: '' Mycosphaerella pinodes'') causes ''Mycosphaerella'' blight. '' Ascochyta pinodella'' (synonym: ''Phoma medicaginis'' var. ''pinodella'') causes ''Ascochyta'' foot rot, and ''Ascochyta pisi'' causes Ascochyta blight and pod spot. Of the three fungi, ''Ascochyta pinodes'' is of the most importance. These diseases are conducive under wet and humid conditions and can cause a yield loss of up to fifty percent if left uncontrolled. The best method to control ascochyta blights of pea is to reduce the amount of primary inoculum through sanitation, crop-rotation, and altering the sowing date. Other methods—chemical control, biological control, and development of resistant varieties—may also be used to effectively control ascochyta diseases.

Host and symptoms

The ascochyta blight disease complex affects field peas (''Pisum sativum''), as well as many other legumes such as chick peas, lentils, and faba beans. Although three different pathogens cause ascochyta diseases of pea, the symptoms are relatively similar to one another, thus making diagnosis difficult. However, there are some small differences between the fungal pathogens.

Mycosphaerella blight (''M. pinodes'')

Ascospores of ''M. pinodes'' produce leaf infections that can be identified by many small purple spots on the undersides of leaves. Under dry conditions, these spots remain small and have no well-defined margin. However, under moist conditions, the purple spots enlarge, turning into well-defined, brown- black lesions. Sometimes these lesions will enlarge and coalesce together forming a completely blighted leaf. The infected leaf will die but will still remain attached to the plant. From the attachment point of infected leaves, purplish-brown stem lesions are produced. These lesions extend upward and downward from the point of attachment. Over time, these lesions become increasingly longer and oftentimes coalesce with to completely girdle the stems of the plant. This gives the lower half of the plant a blue-black appearance. When ''M. pinodes'' infects the blossoms, small, pinpoint lesions appear on the flowers causing the blossom or small pod to drop. This greatly affects the number of surviving pods and limits seed production. ''M. pinodes'' infected seeds might not show symptoms, but if symptoms are present, the seeds may appear shrunken and have a dark-brown discoloration. Planting of infected seeds may result in seedlings with foot rot. Severe infection may kill or stunt young plants and in mature plants, it is likely to cause senescence of all lower leaves and blackening of the stems at the base of the plants.

Ascochyta foot rot (''P. pinodella'')

The symptoms of ''P. pinodella'' are very similar to those caused by ''M. pinodes'', but less severe. ''P. pinodella'' tends to cause less damage to the leaves, stems, and pods. In contrast, the foot rot is usually more severe, oftentimes infecting the stem at the soil line and extending below ground, causing the lateral roots to die.

Ascochyta blight and pod spot (''A. pisi'')

''A. pisi'' can be identified by slightly sunken, tan-colored lesions that are defined by dark-brown margins. Lesions on leaves and pods are circular in shape, while lesions are elongated on stems. Oftentimes, small black pycnidia are present. ''A. pisi'' rarely attacks the base of the plant or causes foot rot in comparison to ''M. pinodes'' and ''P. pinodella''.

Diagnosis

Certain techniques can be used to determine which pathogen is causing disease. One standard technique for distinguishing strains is microscopy. Under a microscope, ''M. pinodes'' can be diagnosed by the presence of pseudothecia. ''P pinodella'' can be diagnosed by the size of conidia produced. ''P. pinodella'' produces conidia that are smaller than the conidia of ''M. pinodes'' or ''A. pisi''. ''A. pisi'' can be diagnosed by the color of the conidia. In comparison to the light colored, buff spore masses of ''M. pinodes'' and ''P. pinodella'' produced on oatmeal agar, ''A. pisi'' spores masses are carrot red.

Other techniques for diagnosis involve serological assays, isoenzyme analysis, restriction fragment length polymorphisms (RFLPs), random amplified polymorphic DNA (RAPD) assays, and by using monoclonal antibodies.

Disease cycle

Ascochyta blight of pea is caused by Ascomycete fungi. This fungus has an anamorphic (asexual) stage and a teleomorphic (sexual) stage. Ascochyta fungal pathogens are heterothallic, meaning they require two compatible hyphae strains to form their sexual stage. Pycnidia of ''Ascochyta'' spp. can overwinter in soil, seeds, or infected plant debris. They release pycnidiospore