Magnaporthe salvinii

Host and Symptoms

Magnaporthe salvinii (telomorph), also known as Sclerotium oryzae (anamorph) and Nakataea sigmoidae (anamorph)^[1], is a ascomycete fungal pathogen that affects a few rice and grass species of plants. These species include:

Echinochola colona – Junglerice

Oryza satvia – Rice

Zizania aquatic – Annual Wildrice

Zizaniopsis milliacea – Giant cutgrass

Turfgrasses

The first symptom that typically appears is a small, black, irregular lesion on the leaf sheath. This lesion will appear near the water line that the sclerotium of the pathogen began its infection. Next, the lesion will grow and penetrate the inner sheath until the leaf sheath becomes rotted partially or in totality.^[2] The culm of the plant then experiences black or brown discoloration and begins to shrivel. When the plant reaches maturity, the stem will break due to the weakened, rotted stem due to infection. A sign of this disease is seeing the sclerotia developing within the dead tissue of the rotted stem.^[3]

Disease Cycle

Magnaporthe salvinii is an ascomycete fungus that infects a plant in an polycyclic disease cycle, through conidia that are tear drop shaped. Once a conidium lands on a leaf, it sticks to the leaf surface and generates a germ tube, appressorium, and penetration peg to bore through the cuticle of the plant. From this, the penetration peg forms a branched feeding structure analogous to haustoria formed in other fungi. As lesions form on the host plant, the pathogen continues to produce condiophores and conidia that may be dispersed by rain splash to infect more plants.^[4] As conditions become unfavorable, the fungus overwinters in sclerotia inside the dead host plant tissue, or within the soil. The sclerotia can be dispersed among soil through rain and water irrigation. Moist, humid conditions, or high nitrogen fertilization are favorable conditions for the plant, in which the sclerotia will germinate in areas of the leaf or sheath near the water line.^[5] Once these sclerotia germinate, the mycelium will infect the plant through wounds or natural openings in the plant. From this, the fungus will produce conidia and the repeat its life cycle.^[6]

Management

One form of control for this pathogen is tilling the field to reduce the number of sclerotia. Maximum tilling of the field significantly reduces the viability of the sclerotia in the soil compared to minimum and no tillage.^[7] Another useful means for eliminating sclerotia in soil is by open field burning, however, overuse of this method in some areas requires other practical means. Other forms of management to reduce the number of sclerotia as inoculum are swathing and fall incorporation with winter flooding. Additionally, controlling the amount of nitrogen fertilizer added to the soil will reduce germination of sclerotia, since they favor high nitrogen soil. Lastly, another important means of control of this fungal pathogen is through the use of fungicides. Quadris is one specific commercial fungicide that is capable of killing the fungal pathogen.^[8]

External links

• Index Fungorum
• USDA ARS Fungal Database

References