Pathogens are infectious agents such as virus, bacteria, prion, fungi, viroid or parasite that causes disease in its host;

Anthracnose, Chlorosis and Necrosis aren't Pathogens

Algal Leaf Spot
Cephaleuros virescens.

Common on Cattleyas but it can infect other orchid genera. Warm weather, rain and high humidity help this disease from spreading. "Splashing" water spreads the spores around and the algae can survive adverse conditions. The spots blemishes noticed on leaves are greenish to pale reddish and have uneven wavy margins. The best way to control the disease is to remove the infected leaves as well as the leaves found on the ground. Good air circulation, good soil drainage and low humidity also help. Spraying with a copper based fungicides a few times, 2-3 weeks apart helps to control and eliminate the algae.

Acidovorax, Erwinia, Pectobacterium, Pseudonomas,
Sclerotium rolfsii
Known as: "Orchid Wilt", "Southern Blight", "Collar Rot", "Sclerotium Rot"
Black Rot
Phytophthora cactorum; Pythium ultimum

Fungal disease capable of killing a plant if left unchecked. In the presence of water, spores penetrate the plant tissue and start their life cycle. The infection causes small, translucent spots, that expand and change to brown or black. The lesions are soft and can secrete “fluids” when squeezed. Seedlings and young leaves are the most vulnerable. Affected leaves may turn yellow around the infected area. Good air circulation is the key to control the fungi. Airflow will dry the leaves and prevent the germination of the spores. Infected leaves and other parts should be removed. Cinnamon powder can be used to dust the wound. Systemic fungicides should be applied to protect the plants.

Bletia Leaf Spot
Volutella concentrica.
Caused by Volutella concentrica, Bletia Leaf Spot is often mistaken for anthracnose (Colletotrichum bletiae). Characteristic of this disease is the formation of large dark "spots", each made up of numerous bluish-colored concentric rings. There is no much information available. Bletia Leaf Blight.?
Fungal disease
Botryodiplodia oncidii
Flower Break
Tobamovirus; Odontoglossum ringspot virus (ORSV)
Schizothyrium perexiguum
Cercospora angreci.
Fusarium Wilt
Sclerotium sp.
Caused by shortage of water during the growing season (shriveled leaves). Same symptoms are visible when root rot kills the roots (due to over watering) and the leaves don't receive the moisture required. The plant can not be saved if there are no live roots.
Gray Mold
Botrytis cinerea; Botrytis fuckeliana
Taphrina (fungus) or Geminiviridae family (virus).
Leaf Rot
Pythium splendens
This disease causes the discoloration of orchid leaves. A spot will appear on a leaf. The spot will become bigger and darker and soft. Cut the leaf back to healthy tissue. Spray the wound with a bactericide or cinnamon. Good air circulation eliminates these problems.
Potexvirus; Cymbidium mosaic virus (CymMV); Tobamovirus (ORSV)
Orchid Fleck Virus (OFV)
Non enveloped, bacilliform particles, sap-transmissible. Causes ring spots (chlorotic or necrotic) and fleck symptoms. Allegedly transmitted by mites.
Petal Blight
Scletotinia fuckeliana; Curvularia eragrostidis; Alternaria alternata;
Brown spots on flower petals, very noticeable on white flowers. Keep plants (flowers) protected from rain. Not to be confused with botrytis.
Powder Mildew
Erysiphaceae family
Fungal disease caused by species of fungi in the order Erysiphales. The symptoms are very distinct: White powdery spots on leaves and stems, appearing at any level above the ground. Given time,the spot size increases and as the numbers of asexual spores are formed increases it becomes denser. The fungi require subtropical conditions to grow (high humidity, moderate temperatures). Can be controlled with systemic fungicides.
Cymbidium Ringspot Virus; Cymbidium Necrotic Ringspot Virus.
Root Rot (Fungal)
Rhizoctonia solani
Rotting of the root caused by Rhizoctonia fungus. Causes the roots to die and may affect the rhizomes. Un pot the infected plant, remove the affected tissue, dip it in fungicide or spray it well, and repot it using a clean pot and potting mix.
Phyllosticta Leaf Spot
Phyllosticta capitalensis (Also known as Guignardia)
Easily spreading fungal disease, causing unsightly blemishes on orchid leaves. Predominantly found in warmer areas. The fungus attacks mostly Dendrobiums and Vandaceous orchids; however, it also infects Cattleyas, Cymbidiums, Phalaenopsis, Oncidiums and other orchid families.
Phytophthora parasitica;
Phytophthora Black Rot
Uredo behnickiana;  Hemileia oncidii; Sphenospora kevorkianii; Coleosporium bletiae
Tomato Spotted Wilt Virus (TSWV); Impatiens Necrotic Spot Virus (INSV); Capsicum Chlorosis Virus (CaCV);
Tospovirus belongs to a Virus family (Bunyaviridae) which predominantly infects animals. The genus Tospovirus is the only plant infecting member. Difficult to detect/prove. The virus is transmitted by Thrips.
White Cell Necrosis
Potexvirus;  (CymMV)