Diseases of White clover (1)

Causal organism: Bean yellow mosaic virus (BYMV), White clover mosaic virus (WCMV), virus (unidentified)
Typical viral disease. Most individuals are infected at the second or third year from seeding in the warm regions. The symptoms begin to appear in spring and are various such as yellow mosaic, green mosaic, chlorotic spots, rugose, etc. according to the varieties of plants and environmental conditions. The virus is transmitted by sucking of the various aphids. The main ones of the causal virus are BYMV and WCMV. The strains of the virus can be roughly estimated by the symptom.

Causal organism: Phytoplasma (PLO)
Phytoplasma disease causing phyllody of head flowers. In Japan, the disease was found in Kanto District, the central region, in 1970's. In mild symptoms, peduncles and calyxes elongate abnormally, but calyxes turned to leaves and elongate from head flowers in severe symptoms. Elongating leaves gradually turned yellow or red from the leaf edge and die in a short period. It also occurs in red clover and alsike clover. The appearance of phyllody increases under warm conditions more than 25C, and it is considered to relate the velocity of phytoplasma proliferation infecting in plant bodies.

Black leaf blight
Causal organism: Leptotrochila trifolii Narita, Ascomycotina
Fungal disease which causes leaf blight occurring in the cool region. The lesion is formed from the leaf rim and becomes black brown and V letterform with yellow surroundings. The small black grains (perithecia) are formed on the surface of the lesion, and ascospores are dispersed by wind and rain. The causal organism can infect other clovers, but their pathogenicities are differentiated.

Common leaf spot
Causal organism: Pseudopeziza trifolii (Bivona-Bernardi) Fuckel, Ascomycotina
Typical spot-causing fungal disease of clover. Since the disease occurs severely in cool and wet conditions, the damage is large in Hokkaido, the northern part of Japan. Many brown to blackish brown spots are at first produced and enlarge gradually, but they often remain 1-3 mm in diameter. Dark brown perithecia like warty are produced in the center part of the lesion later and this part becomes like jelly at wet conditions and disperses spores. A lot of leaves are diseased and wither when occurring severely. The species of the causal organism is different from common leaf spot fungi of alfalfa. The disease occurs in white clover and alsike clover, too.

Curvularia leaf blight
Causal organism: Curvularia trifolii (Kauffman) Boedijn, Imperfect fungi
Fungal disease which causes large damage especially in white clover. The lesions at first appear as yellow discolored parts, and soon they turn to brown and become characteristic V letterform lesions divided by the leaf vein. When the petiole is infected, the leaf withers and are killed soon. When occurring severely, the whole plant looks like burned and the yield decreases much. Damage is large in younger leaves than old ones. Ladino-clover is particularly weak to the disease. It occurs severely when the temperature increases considerably after the rainy season. The pathogen infects other clovers.

Leaf curl
Causal organism: Olpidium trifolii Schroter, Mastigomycotina
Fungal disease to cause swellings like blisters in leaf. The lesions are 2-5 mm in diameter in the leaf and do not discolor. They become like galls when occurring in the stalk and petiole and become screwed bending in the part. The plant vigor declines so much when occurring severely. It occurs only in white clover.

Leaf spot
Causal organism: Stagonospora recedens (Massalongo) Jones et Weimer, Imperfect fungi
Spot-causing fungal disease mainly occurring in the leaf. The lesions are dark brown with grayish white borders, oval to round and about 2-5 mm in length. Faint zonation might be formed in the lesion and small black grains (pycnidia) are produced on the surface. The lesions fuse mutually and the leaf withers. The causal organism infect red clover, too.

Leaf rust
Causal organism: Uromyces trifolii-repentis Liro, Basidiomycotina
Rust disease occurring in the leaf. The uredinia are drab to brown, round, upheaving a little and scatters in the whole leaf. The telia are dark brown and upheave a little. The leaf withers rolling up when occurring severely.

Pepper spot
Causal organism: Leptosphaerulina trifolii (Rostrup) Petrak, Ascomycotina
Spot-causing fungal disease occurring in the cool regions. The blackish brown, small lesions of 1-2 mm in diameter are produced a lot and the whole leaf looks like sprinkled with black pepper. The surroundings of the lesion turn to yellow gradually and the leaf withers finally. It occurs severely in the cool condition with frequent rain. Black small grains (perithecia) are formed on the old lesions. The species of the causal organism is different from pepper spot fungi of alfalfa.

Powdery mildew
Causal organism: Microsphaera trifolii (Greville) Brown (=Erysiphe trifolii Greville), Ascomycotina
Fungal disease which makes plants white powdery. White, thin and indistinct colonies like sprinkled white powder are produced on the surface of the leaf in the early spring. They gradually become thick, cover the whole leaf and cause leaf blight. The disease occurs severely in the cool and dry condition, and the damage becomes large when sunshine is insufficient under the cloudy weather. The causal organism can infect wide ranges of legumes such as red clover, alsike clover and vetches.

Root rot
Causal organism: Rhizoctonia solani Kuhn, Basidiomycota
Fingal disease that makes plants kill. In Japan, the disease was first reported in the Hokuriku region. In warm conditions, it causes summer blight, due to the rot of the tap root of 5-10cm under the crown. Root rot gradually progress to the lower part of the root, resulting the whole root rot of the infected plants. When occurring severely, the upper-ground parts of the plants are killed, resulting summer blight. The symptom becomes more severe if the harvest by cutting the palnts frequently. In Hokkaido, the northern region of Japan, Fusarium solani also causes same symptoms.

Sclerotinia crown and stem rot
Causal organism: Sclerotinia trifoliorum Eriksson, Ascomycotina
Important fungal disease which causes plant death occurring in cool and wet regions. The small spots appears at first and then the leaf and stalk turn to yellow and wither. The disease progresses gradually under the snowfall. The stem, leaf and root rot to ash white according to increase of the temperature after snow-melting in the spring of next year. White and fluffy hyphae are produced on the surface of the withering plant and large black sclerotia of irregular types and about 8-10mm in size are produced before long. They germinate in autumn and produce light brown stroma of 3-8 mm in diameter. The ascospores spread from the stroma and the infection happens again. The host range of the causal organism is wide including alfalfa and vetches.

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