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Symbiotic interactions
Ascomycetes have been documented to have many symbiotic relationships with both plants and animals. Some of these symbioses are beneficial, other interactions are detrimental, and many are not currently fully understood.

Symbiotic relationships with animals
Several ascomycetes of the genus Xylaria colonize the nests of leafcutter ants and other fungus-growing ants of the tribe Attini, and the fungal gardens of termites (Isoptera). The fungi that colonize the higher Attini are obligate mutualists that do not produce spores and cannot survive outside the colony.

Bark beetles (family Scolytidae) are important symbiotic partners of ascomycetes. The female beetles transport fungal spores to new hosts in characteristic tucks in their skin, the mycetangia. The beetle tunnels into the wood and into large chambers in which they lay their eggs. Spores released from the mycetangia germinate into hyphae, which can break down the wood. The beetle larvae then feed on the fungal mycelium, and, on reaching maturity, carry new spores with them to renew the cycle of infection. A well-known example of this is Dutch elm disease, caused by Ophiostoma ulmi, which is carried by the European elm bark beetle, Scolytus multistriatus.

Plant disease interactions
One of their most harmful roles is as the agent of many plant diseases. For instance:


 * Dutch Elm Disease, caused by the closely related species Ophiostoma ulmi and Ophiostoma novo-ulmi, has led to the death of many elms in Europe and North America.


 * The originally Asian Cryphonectria parasitica is responsible for attacking Sweet Chestnuts (Castanea sativa), and virtually eliminated the once-widespread American Chestnut (Castanea dentata),
 * A disease of maize (Zea mays), which is especially prevalent in North America, is brought about by Cochliobolus heterostrophus.
 * Taphrina deformans causes leaf curl of peach.
 * Uncinula necator is responsible for the disease powdery mildew, which attacks grapevines.
 * Species of Monilinia cause brown rot of stone fruit such as peaches (Prunus persica) and sour cherries (Prunus ceranus).
 * Members of the Ascomycota such as Stachybotrys chartarum are responsible for fading of woolen textiles, which is a common problem especially in the tropics.
 * Blue-green, red and brown molds attack and spoil foodstuffs - for instance Penicillium italicum rots oranges.
 * Cereals infected with Fusarium graminearum contain mycotoxins like deoxynivalenol (DON), which causes Fusarium ear blight and skin and mucous membrane lesions when eaten by pigs.

Human disease interactions

 * Aspergillus fumigatus, the most common cause of fungal infection in the lungs of immune compromised patients often resulting in death. Also the most frequent cause of Allergic bronchopulmonary aspergillosis, which often occurs in patients with Cystic fibrosis as well as Asthma.
 * Candida albicans, a yeast that attacks the mucous membranes, can cause an infection of the mouth or vagina called thrush or candidiasis, and is also blamed for "yeast allergies".
 * Fungi like Epidermophyton cause skin infections but are not very dangerous for people with healthy immune systems. However, if the immune system is damaged they can be life-threatening; for instance, Pneumocystis jirovecii is responsible for severe lung infections that occur in AIDS patients.
 * Ergot (Claviceps purpurea) is a direct menace to humans when it attacks wheat or rye and produces highly poisonous and carcinogenicalkaloids, causing ergotism if consumed. Symptoms include hallucinations, stomach cramps, and a burning sensation in the limbs ("Saint Anthony's Fire").
 * Aspergillus flavus, which grows on peanuts and other hosts, generates aflatoxin, which damages the liver and is highly carcinogenic.