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Common Humidity diseases

Eliminating Humidity Related Diseases Through Dehumidification

Humidity related diseases and molds, such as botrytis cinerea (gray mold, bud rot) and powdery mildew, are commonly found in commercial greenhouse and indoor cultivation. These diseases affect nearly all crops including vegetables, fruit, herbs, ornamentals and cannabis, and may lead to severe yield loss and diminished output quality.

All diseases found below require high levels of humidity, even briefly, in order to breakout. High relative humidity in a growing space may be found due to uneven climate conditions, fluctuations in humidity and temperature, such as during dusk or dawn and a lack of air circulation.

Click for further information about common humidity problems and diseases typical in commercial greenhouses and growing facilities.

DISEASES

This page provides information on the humidity diseases most commonly encountered by our growers.

Powdery Mildew

Powder mildews consist of variety of fungal species which infect and cause diseases in many different species of plants. These diseases get their name from powder spores that appear on infected leaf surfaces. Symptoms usually begin on the older, lower leaves. As their capacity to conduct photosynthesis is hurt, plants may lose their color. Leaves may wither and die as the disease progresses. Powder mildew symptoms and favorable conditions vary between species.

Powdery Mildew in Solanaceous Crops

Different species of powdery mildew-causing fungi will produce different symptoms on solanaceous crops. Some species will cause the appearance of yellow or bright green spots on the upper sides of leaves. Other species will cause whitish-greyish powdery coating to appear on the upper sides of leaves. powdery coating may be found on the undersides of leaves.
white spots on tomato plant tomato powdery mildew

White spots on tomato plant

“Tomato:Powdery mildew” by scot nelson, licensed under CC BY 2.0

Powdery Mildew sexual spores on the surface of a leaf

Yellowing foliar spots on pepper plant

Powdery Mildew in Cucurbits

White powdery spots will appear first on the undersides of leaves , and then the upper sides of leaves, stems, and petioles. Late in the growing season, small dark brown spherical structures may develop on the leaves – these protect the fungal spores during the winter and may become sources of infection in following seasons.
White Powdery mildew spots on cucumber leaf

White spots on cucumber leaf

“Cucurbit Powdery mildew” by scot nelson, licensed under CC BY 2.0

Powdery Mildew spores on a lower surface of a leaf

Sexual spores on the lower surface of leaf

University of Georgia Plant Pathology , University of Georgia, Bugwood.org , licensed under CC 3.0

Powdery Mildew in Cannabis

White powdery spots will appear on leaf surfaces , petioles , and flowers. As the powdery mildew patched age , small spherical structures may appear and will change colors from yellow to black as they age which carry overwintering spores.

Late Blight

Late blight can severely damage plants, and attacks fast. This disease attacks tomatoes and potatoes. It is characterised by black/brown lesions on leaves and stems that expand rapidly and became necrotic. White “fuzz” (spores) can be seen around the lesions. These spots can also be seen on the fruit, and are often followed by soft rot and disintegration. The entire plant can be destroyed in only a few days after the first lesions are observed. This is the plant disease that led to the “Irish Potato Famine” from 1849 – 1845 , where damages to potato crops caused yield losses that led to mass starvation in Ireland.

White late blight spores on tomato fruit lesions

White spores on tomato fruit lesions

“Tomato: Late blight” by Scot Nelson is licensed under CC BY 2.0

late blight on tomato stem

Brown lesions on tomato stem

“Late blight on tomato stem” by Dwight Sipler is licensed under CC BY 2.0

Late blight tomato fruit lessions

Tomato fruit lesions

Scot Nelson, Tomato late blight fruit cluster

Late blight tomato

Late blight epidemic on tomato plant

“Tomato Late Epidemic” by Scot Nelson, licensed under CC BY 2.0

Bacterial canker on tomato

Bacterial Canker on Tomato

Scot Nelson licensed under CC BY 2.0

Late blight lessions tomato stem

Brown lesions on tomato stem

Late blight of tomato (Lycopersicon esculentum) in a garden near Hilo, Hawaii, caused by Phyophthora infestans” by Scot Nelson licensed under CC BY 2.0

Brown tomato stem late blight

Brown discolouration in stem centre of tomato plant

brown late blight tomato

Brown discolouration in stem centre of tomato plant

Downy Mildew

This disease can be recognized by the gray, dark spores that grow on the under sides of leaves. The fungus can grow systemically through the plant. Small yellow spots may develop on the upper sides of leaves. Infected leaves and branches may become distorted and die. The disease favors cool temperatures and high humidity. Note: Downy mildew can sometimes be confused for powdery mildew or botrytis.
Yellow Downy Mildew spots on upper side of cucumber leaf

Yellow spots on upper side of cucumber leaf

Yellowing and browning of basil leaf because of Downy Mildew

Yellowing and browning of basil leaf

“Basil (Ocimum basilicum): Downy mildew, caused by Peronospora belbahrii” by Scot Nelson is licensed under CC BY 2.0

Yellowing and browning of basil leaf Downy Mildew

Dark spores on lower side of cucumber leaf

Downy Mildew Spores on lower surface of basil leaf

Spores on lower surface of basil leaf

“Sweet basil (Ocimum basilicum): Downy mildew, caused by Peronospora belbahrii” by Scot Nelson is licensed under CC BY 2.0

Dark Downy Mildew spores and withering leaves on basil leaf

Brown lesions on tomato stem

Late blight of tomato (Lycopersicon esculentum) in a garden near Hilo, Hawaii, caused by Phyophthora infestans” by Scot Nelson licensed under CC BY 2.0

Yellow Dark spores and withering leaves on basil leaf

Yellow spots on upper side of cucumber leaf

Alternaria

Alternaria species produce diseases whose most common symptom is circular spots on leaves with target like, circular rings. These spots can be yellow, dark brown, and black and can be found on all parts of the plants. Black speckles are often found on these spots which are the spores. The centers of these spots may fall out, giving the leaf spots a shot-hole appearance. Individual spots can connect into large dead areas, and leaves may fall. Sometimes, as with tomatoes or potatoes, it also attacks fruits, producing black round or oval spots and spoiling crops. Although Alternaria normally develops on the leaves, it can also attack the stems of young seedlings, creating spots of grayish-brown color (normally at the base of the stem) causing rot that often kills the plant.Note: Downy mildew can sometimes be confused for powdery mildew or botrytis.
Alternaria Circular, target-like lesions on tomato leaf

Circular, target-like lesions on tomato leaf

“A tomato leaf showing typical target shaped lesions of Alternaria solani”

Alternaria - humidity disease, Brassica family

“Shot holes” – lesions with centers that fell out, Brassica family

Gerald Holmes, California Polytechnic State University at San Luis Obispo, Bugwood.org, licensed under a Creative Commons Attribution-Noncommercial 3.0 License.

Lesions and black spores on tomato fruit Alternaria

Lesions and black spores on tomato fruit

Yuan-Min Shen, Taichung District Agricultural Research and Extension Station, Bugwood.org, licensed under CC 3.0.

Alternaria rot on stems young tomato seedlings Alternaria

Alternaria rot on stems young tomato seedlings

Virginia Tech Learning Resources Center, Virginia Polytechnic Institute and State University, Bugwood.org, licensed under CC 3.0

Alternaria alternata fungus on cannabis flower

Alternaria alternate fungus on cannabis flower

Gray Mold – Botrytis Cinerea

There are over 200 species of Botrytis, a very common greenhouse humidity disease. In conditions of high relative humidity, botrytis penetrates the plant and develops. It can be found on leaves, stems, flowers, and fruit. The tissue on which it develops becomes dark and sometimes soft, due to the death of the host cells. Botrytis infections can be recognized by the appearance of gray, dusty spores with a velvety appearance. In some flowers, light colored spots with a dark brown ring can indicate mold infection. Once the infection is visible to the naked eye, the mold has already penetrated the plant and chemical treatments often will not have effective results. This disease is a major problem for most greenhouse growers.

There are over 200 species of Botrytis , a very common greenhouse humidity disease. In conditions of high relative humidity, botrytis penetrates the plant and develops. It can be found on leaves, stems, flowers, and fruit.

The tissue on which it develops becomes dark and sometimes soft, due to the death of the host cells. Botrytis infections can be recognized by the appearance of gray, dusty spores with a velvety appearance. In some flowers, light colored spots with a dark brown ring can indicate mold infection. Once the infection is visible to the naked eye, the mold has already penetrated the plant and chemical treatments often will not have effective results. This disease is a major problem for most greenhouse growers.

Gray Mold Botrytis spores on eggplant

Gray spores on eggplant

Gray Mold Spores on tomato stem

Spores on tomato stem

Botrytis symptoms on cannabis flower

Botrytis symptoms on cannabis flower

Gray mold Botrytis generates white ring on tomato fruit

Botrytis generates white ring on tomato fruit

by golglocki , CC - BYSA3.0

Grey mold spores at the base of campanula flowers

Gray spores at the base of campanula flowers

Damaged tomato stem by gray mold

Damage on tomato stem

Infected Cannabis flower with botrytis gray grey mold

Infected cannabis flower

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