Mold (American English) or mould (British English) is part of the natural environment. Outdoors, molds play a part in nature by breaking down dead organic matter such as fallen leaves and dead trees; indoors, mold growth should be avoided. Molds reproduce by means of tiny spores. The spores are invisible to the naked eye and float through the air. Mold may begin growing indoors when spores land on moist surfaces. There are many types of mold, but all require moisture for growth.

Symptoms of mold exposure may include nasal and sinus congestion; runny nose, eye irritation; itchy, red, watery eyes, respiratory problems, such as wheezing and difficulty breathing, chest tightness, cough, throat irritation, skin irritation (such as a rash), headache, and persistent sneezing.

Snow mold is a type of fungus and a turf disease that damages or kills grass after snow melts, typically in late winter. Its damage is usually concentrated in circles three to twelve inches in diameter, although yards may have many of these circles, sometimes to the point at which it becomes hard to differentiate between different circles. Snow mold comes in two varieties: pink or gray. While it can affect all types of grasses, Kentucky bluegrass and fescue lawns are least affected by snow mold.

Cladosporium fulvum is an Ascomycete called "Passalora fulva", a non-obligate pathogen that causes the disease on tomato known as the Tomato leaf mold. P. fulva only attacks tomato plants, especially the foliage, and it is a common disease in the greenhouses, but can also occur in the field. The pathogen is likely to grow in humid and cool conditions. In the greenhouses, this disease causes big problems during the fall, in the early winter and spring, due to the high relative humidity of air and the temperature, that are propitious for the leaf mold development. This disease was first described in the North Carolina, by Mordecai Cubitt Cooke (1883), on cultivated tomato (Cooke 1883), although it is original from South and Central America. The causal fungus of tomato leaf mold may also be referred as Cladosporium fulvum (Cooke 1883), a former name.

Initial symptoms include large, angular or blocky, yellow areas visible on the upper surface. As lesions mature, they expand rapidly and turn brown. The under surface of infected leaves appears watersoaked. Upon closer inspection, a purple-brown mold (see arrow) becomes apparent. Small spores shaped like footballs can be observed among the mold with a 10x hand lens. In disease-favorable conditions (cool nights with long dew periods), downy mildew will spread rapidly, destroying leaf tissue without affecting stems or petioles.

Common spot of strawberry is one of the most common and widespread diseases of strawberry. Common spot of strawberry is caused by the fungus Mycosphaerella fragariae (imperfect stage is "Ramularia tulasnei"). Symptoms of this disease first appear as circular, dark purple spots on the leaf surface. "Mycosphaerella fragariae" is very host specific and only infects strawberry.

Mycosphaerella fragariae is a species from family Mycosphaerellaceae.

Downy mildew refers to any of several types of oomycete microbes that are obligate parasites of plants. Downy mildews exclusively belong to Peronosporaceae. In commercial agriculture, they are a particular problem for growers of crucifers, grapes and vegetables that grow on vines. The prime example is "Peronospora farinosa" featured in NCBI-Taxonomy and HYP3. This pathogen does not produce survival structures in the northern states of the United States, and overwinters as live mildew colonies in Gulf Coast states. It progresses northward with cucurbit production each spring. Yield loss associated with downy mildew is most likely related to soft rots that occur after plant canopies collapse and sunburn occurs on fruit. Cucurbit downy mildew only affects leaves of cucurbit plants.

Physiological plant disorders are caused by non-pathological conditions such as poor light, adverse weather, water-logging, phytotoxic compounds or a lack of nutrients, and affect the functioning of the plant system. Physiological disorders are distinguished from plant diseases caused by pathogens, such as a virus or fungus. While the symptoms of physiological disorders may appear disease-like, they can usually be prevented by altering environmental conditions. However, once a plant shows symptoms of a physiological disorder it is likely that that season’s growth or yield will be reduced.

Gray snow mold ("Typhula" spp. or Typhula blight) is the less damaging form of snow mold. While its damage may appear widespread, it typically does little damage to the grass itself, only to the blades. Unlike most plant pathogens, it is able to survive throughout hot summer months as sclerotia under the ground or in plant debris. Typhula blight is commonly found in United States in the Great Lakes region and anywhere with cold winter temperatures and persistent snow fall.

This disease affects strawberry plant foliage causing purple spots ⅛ to ¼ inches across on the upper side of the leaves. At first, the whole spot is purple but as the disease matures the center of the leaf spots become tan or gray, then almost white. When numerous spots merge foliage death can occur; this can stunt or kill infected plants when severe.

On petioles, stolons, calyxes, and fruit trusses, elongated lesions may form and interfere with water transport in the plant, weakening the plant and making it more susceptible to invasion by a secondary organism.

Blight refers to a specific symptom affecting plants in response to infection by a pathogenic organism. It is a rapid and complete chlorosis, browning, then death of plant tissues such as leaves, branches, twigs, or floral organs. Accordingly, many diseases that primarily exhibit this symptom are called blights. Several notable examples are:

- Late blight of potato, caused by the water mold "Phytophthora infestans" (Mont.) de Bary, the disease which led to the Great Irish Famine

- Southern corn leaf blight, caused by the fungus "Cochliobolus heterostrophus" (Drechs.) Drechs, anamorph "Bipolaris maydis" (Nisikado & Miyake) Shoemaker, incited a severe loss of corn in the United States in 1970.

- Chestnut blight, caused by the fungus "Cryphonectria parasitica" (Murrill) Barr, has nearly completely eradicated mature American chestnuts in North America.

- Fire blight of pome fruits, caused by the bacterium "Erwinia amylovora" (Burrill) Winslow "et al.", is the most severe disease of pear and also is found in apple and raspberry, among others.

- Bacterial leaf blight of rice, caused by the bacterium "Xanthomonas oryzae" (Uyeda & Ishiyama) Dowson.

- Early blight of potato and tomato, caused by species of the ubiquitous fungal genus "Alternaria"

- Leaf blight of the grasses

On leaf tissue, symptoms of blight are the initial appearance of lesions which rapidly engulf surrounding tissue. However, leaf spot may, in advanced stages, expand to kill entire areas of leaf tissue and thus exhibit blight symptoms.

Blights are often named after their causative agent, for example Colletotrichum blight is named after the fungi "Colletotrichum capsici", and Phytophthora blight is named after the water mold "Phytophthora parasitica".

Mold health issues are potentially harmful effects of molds.

Molds (US usage; British English "moulds") are ubiquitous in the biosphere, and mold spores are a common component of household and workplace dust. The United States Centers for Disease Control and Prevention reported in its June 2006 report, 'Mold Prevention Strategies and Possible Health Effects in the Aftermath of Hurricanes and Major Floods,' that "excessive exposure to mold-contaminated materials can cause adverse health effects in susceptible persons regardless of the type of mold or the extent of contamination." When mold spores are present in abnormally high quantities, they can present especially hazardous health risks to humans after prolonged exposure, including allergic reactions or poisoning by mycotoxins, or causing fungal infection (mycosis).

Zygomycosis is the broadest term to refer to infections caused by "bread mold fungi" of the zygomycota phylum. However, because zygomycota has been identified as polyphyletic, and is not included in modern fungal classification systems, the diseases that zygomycosis can refer to are better called by their specific names: mucormycosis (after Mucorales), phycomycosis (after Phycomycetes) and basidiobolomycosis (after Basidiobolus). These rare yet serious and potentially life-threatening fungal infections usually affect the face or oropharyngeal (nose and mouth) cavity. Zygomycosis type infections are most often caused by common fungi found in soil and decaying vegetation. While most individuals are exposed to the fungi on a regular basis, those with immune disorders (immunocompromised) are more prone to fungal infection. These types of infections are also common after natural disasters, such as tornadoes or earthquakes, where people have open wounds that have become filled with soil or vegetative matter.

The condition may affect the gastrointestinal tract or the skin. In non-trauma cases, it usually begins in the nose and paranasal sinuses and is one of the most rapidly spreading fungal infections in humans. Common symptoms include thrombosis and tissue necrosis. Treatment consists of prompt and intensive antifungal drug therapy and surgery to remove the infected tissue. The prognosis varies vastly depending upon an individual patient's circumstances.

The tomato leaf mold fungus is a specific pathogen of tomato plant Lycopersicon, this pathogen has restricted host range (host specific pathogen) that only infects tomatoes, mainly greenhouses.

The symptoms of this disease commonly occurs on foliage, and it develops on both sides of the leaf on the adaxial and abaxial surface. The older leaves are infected first and then the disease moves up towards young leaves.

Symptoms of tomato leaf mold appear usually with foliage, but fruit infection is rare. The primary symptom appear on the upper surface of infected leaves as a small spot pale green or yellowish with indefinite margins, and on corresponding area of the lower surface, the fungus begins to sporulate. The diagnostic symptom develops on lower surface as an olive green to grayish purple and velvety appearance, which are composed of spores (conidia). Continuously, the color of the infected leaf changes to yellowish brown and the leaf begins to curl and dry. The leaves will drop upon reaching a premature stage, and the defolication of the infected host will cause further infection. This disease develops well in relatively humidity levels (above 85%). When the temperature reaches optimum level for germinating, the host will be infected by the pathogen. Occasionally, this pathogen causes disease on the fruit or blossoms with various symptoms. Fruits such as green and ripe one will develop dark rot on the stem. The blossoms will be killed before fruits grow.

Symptoms of mold exposure can include:

- Nasal and sinus congestion, runny nose

- Respiratory problems, such as wheezing and difficulty breathing, chest tightness

- Cough

- Throat irritation

- Sneezing / Sneezing fits

Cork is often harvested from the cork oak ("Quercus suber") and stored in slabs in a hot and humid environment until covered in mold. Cork workers may be exposed to organic dusts in this process, leading to this disease.

Suberosis, also known as corkhandler's disease or corkworker's lung, is a type of hypersensitivity pneumonitis usually caused by the fungus "Penicillium glabrum" (formerly called "Penicillum frequentans") from exposure to moldy cork dust. "Chrysonilia sitophilia", "Aspergillus fumigatus", uncontaminated cork dust, and "Mucor macedo" may also have significant roles in the pathogenesis of the disease.

Pathogenic zygomycosis is caused by species in two orders: Mucorales or Entomophthorales, with the former causing far more disease than the latter. These diseases are known as "mucormycosis" and "entomophthoramycosis", respectively.

- Order Mucorales (mucormycosis)

- Family Mucoraceae

- "Absidia" ("Absidia corymbifera")

- "Apophysomyces" ("Apophysomyces elegans" and "Apophysomyces trapeziformis")

- "Mucor" ("Mucor indicus")

- "Rhizomucor" ("Rhizomucor pusillus")

- "Rhizopus" ("Rhizopus oryzae")

- Family Cunninghamellaceae

- "Cunninghamella" ("Cunninghamella bertholletiae")

- Family Thamnidiaceae

- "Cokeromyces" ("Cokeromyces recurvatus")

- Family Saksenaeaceae

- "Saksenaea" ("Saksenaea vasiformis")

- Family Syncephalastraceae

- "Syncephalastrum" ("Syncephalastrum racemosum")

- Order Entomophthorales (entomophthoramycosis)

- Family Basidiobolaceae

- "Basidiobolus" ("Basidiobolus ranarum")

- Family Ancylistaceae

- "Conidiobolus" ("Conidiobolus coronatus/Conidiobolus incongruus")

Diagnosis of the cause of a physiological disorder (or disease) can be difficult, but there are many web-based guides that may assist with this. Examples are: "Abiotic plant disorders: Symptoms, signs and solutions"; "Georgia Corn Diagnostic Guide"; "Diagnosing Plant Problems" (Kentucky); and "Diagnosing Plant Problems" (Virginia).

Some general tips to diagnosing plant disorders:

- Examine where symptoms first appear on a plant—on new leaves, old leaves or all over?

- Note the pattern of any discolouration or yellowing—is it all over, between the veins or around the edges? If only the veins are yellow deficiency is probably not involved.

- Note general patterns rather than looking at individual plants—are the symptoms distributed throughout a group of plants of the same type growing together. In the case of a deficiency all of the plants should be similarly effected, although distribution will depend on past treatments applied to the soil.

- Soil analysis, such as determining pH, can help to confirm the presence of physiological disorders.

- Consider recent conditions, such as heavy rains, dry spells, frosts, etc., may also help to determine the cause of plant disorders.

Crayfish plague, "Aphanomyces astaci", is a water mold that infects crayfish, most notably the European "Astacus" which dies within a few weeks of being infected. When experimentally tested, species from Australia, New Guinea and Japan were also found to be susceptible to the infection.

The most common symptom of a fungal nail infection is the nail becoming thickened and discoloured: white, black, yellow or green. As the infection progresses the nail can become brittle, with pieces breaking off or coming away from the toe or finger completely. If left untreated, the skin underneath and around the nail can become inflamed and painful. There may also be white or yellow patches on the nailbed or scaly skin next to the nail, and a foul smell. There is usually no pain or other bodily symptoms, unless the disease is severe. People with onychomycosis may experience significant psychosocial problems due to the appearance of the nail, particularly when fingers – which are always visible – rather than toenails are affected.

Dermatophytids are fungus-free skin lesions that sometimes form as a result of a fungus infection in another part of the body. This could take the form of a rash or itch in an area of the body that is not infected with the fungus. Dermatophytids can be thought of as an allergic reaction to the fungus.

Onychomycosis, also known as tinea unguium, is a fungal infection of the nail. This condition may affect toenails or fingernails, but toenail infections are particularly common.

Treatment may be based on the signs. Treatment may be with the medication terbinafine.

It occurs in about 10 percent of the adult population. It is the most common disease of the nails and constitutes about half of all nail abnormalities.

The term is from Ancient Greek ὄνυξ "ónux" "nail", μύκης "múkēs" "fungus" and -ωσις "ōsis" "functional disease."

In both healthy and immunocompromised hosts, Majocchi's granuloma often presents as nodules and papules on areas that are most exposed to mechanical abuse—wear and tear—such as the upper and lower extremities. Patients will complain about papules, pustules, or even plaques and nodules at the site of infection. The papules will be pink-red and will be located in a perifollicular location. Hair shafts can be easily removed from the pustules and papules. Itching is also very common. Firm or fluctuant subcutaneous nodules or abscesses represent a second form of MG that is generally observed in immunosuppressed hosts. Nodules may develop in any hair-bearing part of the body but are most often observed on the forearms, hands, and legs of infected individuals. Involvement of the scalp and face is rarely observed. Lesions start as solitary or multiple well-circumscribed perifollicular papulopustules and nodules with or without background erythema and scaling. In rare circumstances, the lesions may have keloidal features.

Farmer’s lung reactions can be categorized as acute and chronic reactions. Acute and chronic reactions have the same symptoms but for chronic reactions, the symptoms are much more severe. Farmer’s lung symptoms include:

- Chills

- Fever

- Irritating/harassing cough

- Runny nose

- Sputum streaked with blood

- Tightness of the chest

- Difficult and laboured breathing

- Crackling of breath

- Muscular pain

- Depression

These symptoms develop between four and eight hours after exposure to the antigens. In acute attacks, the symptoms mimic pneumonia or flu. In chronic attacks, there is a possibility of the victim going into shock and dying from the attack.

Farmer's lung (not to be confused with silo-filler's disease) is a hypersensitivity pneumonitis induced by the inhalation of biologic dusts coming from hay dust or mold spores or any other agricultural products. It results in a type III hypersensitivity inflammatory response and can progress to become a chronic condition which is considered potentially dangerous.