Stromatinia cepivora

Sclerotium cepivorum is the asexual reproductive form of Stromatinia cepivora and is a plant pathogen, causing white rot in Allium species, particularly onions, leeks, and garlic.[3] On a worldwide basis, white rot is probably the most serious threat to Allium crop production of any disease.[4] This is a soil borne fungus and affects susceptible crops planted in infected soil containing sclerotia. The sclerotia that are developed in the life cycle can be spread to other fields by unsuccessful sanitation practices.The sclerotia can remain viable in the soil for years and germinate with a susceptible host to cause disease; therefore it is important to practice good sanitation efforts.[5] Where the disease has occurred, recropping with further Allium species should be avoided for many years. The risk of infection can be reduced as far as possible in clean land by using disease-free planting material and avoiding contamination from infected fields. Making sure to clean machinery, boots and equipment will help to stop the spread of disease from an infected field. With infection occurring in cooler weather (50–70 °F (10–21 °C)), planting the crops at the right time is also important to not institute disease.[6]

The first symptoms noted with S. cepivora are the foliar symptoms. Plants are stunted in growth with yellow and wilting foliage. The leaves eventually die and fall off [7] with the older leaves dying first and then the aerial leaves. Soil conditions and the environment are determinants for extent of damage to the plant. The pathogen grows in moist cold temperatures.[8] So, in the right conditions, pathogenic activity increases as the root systems develop. The disease attacks at all stages of growth, which leaves the plant to turn yellow and wilt when fully developed because the roots are rotting. Mycelial growth is another symptom that appears on the roots and spreads to the bulb causing it to rot. This mycelial growth can be seen at the base of the stem when foliage is yellowing and the foliar symptoms are first appearing. Black globular sclerotia, that resemble poppy seeds can also appear on the mycelium. These survival structures (sclerotia) can detach and persist for years in a dormant state, waiting for a susceptible host.[7]

The White Rot pathogen is dependent upon temperature. Environmental conditions influence the germination with it favoring cooler weather (50-70 F).[6] If there is high soil moisture present, germination and infection will be favored. However, the sclerotia and fungal growth are inhibited above 70 F.[9] With the pathogen favoring cool wet summers, irrigation can also be a problem in spreading the disease from an infected field to a clean field. Therefore, this pathogen is of great concern to growers experiencing cool wet summers.

Sclerotia production in S. Cepivora

Stromatinia cepivora is a soil borne fungus. This is a monocyclic disease meaning it only has one reproductive cycle a season. This is a unique fungus as it does not produce any spores of importance to a normal life cycle.[9] It exists and overwinters as sclerotia (the survival stage). These small black globular structures are resistant to adverse temperatures and can remain dormant in the soil for years even without a host. Sclerotia germinate in response to root exudates. Weather is also a factor of germination and hyphae growth. Mycelium grow through the soil and form an appresoria once a host root is available. Appresoria are able to attach and penetrate the host. Mycelium grow out from the roots and can spread to a neighboring plant which creates the row of disease. Even small amounts of sclerotia can cause disease and be difficult to control. Sclerotia infect the host and spread. They are formed on the decaying host tissue and then are left free in the soil.[10] To control the disease there needs to be a reduction in the number of sclerotia in the soil so fungus growth can be halted and unable to grow. Overall, multiple controls are necessary to produce an adequate yield in infected fields.

Anything that moves the infested soil, such as wind, water, equipment, boots, etc., will move sclerotia and cause the disease to spread.

This is serious disease for plants of the Allium genus. The soil borne fungus can persist in the soil for many years. This disease is present in all Allium-producing regions making it a threat in the Allium production industry and a worldwide disease. It has been found in the United States 10 times with the first in 1918 in Oregon and the latest in 2014 in an onion field.[8] Onions and garlic are economically important vegetables in the world. S. cepivora is one of the most destructive diseases carrying high loss in onion and garlic. Once land has been infested, it is considered not suitable for garlic or onion production for up to 40 or more years.[11]

White rot in the United States[8]

• 1918: First found in La Grande, Oregon
• 1930s: San Francisco area
• 1940s: Gilroy and Tulelake, California; Klamath Falls, Oregon; Walla Walla, Washington
• 1950s: Salinas, Nevada; Willamette Valley, Oregon
• 1970s: Central Oregon, San Joaquin Valley
• 1989: Treasure Valley
• 2004: Marion County, Oregon
• 2008: Crook County, Oregon
• 2010: Home-grown garlic in the Palouse Falls region

The three-cornered leek (Allium triquetrum) has been introduced into Australia where it has spread and become established in nutrient-deficient, damp habitats. The plant is now considered to be a noxious invasive species, as it is difficult to control or eradicate. S. cepivora is being investigated as a possible biological control agent for the plant. No naturally occurring members of the genus Allium occur in Australia, and in a trial, the fungus was found to be effective at killing all but one of the target samples on which it was tested. However, the researchers involved in the study acknowledged, "Releasing a virulent pathogen for cultivated Allium species into bushland or pasture is controversial and any field release would require safeguards against spread to areas suitable for the production of cultivated Allium species, such as onions, leeks and garlic, before S. cepivora could be introduced as a potential biological control agent.[15]