Is Powdery Mildew Systemic?

Powdery Mildew is a fungal disease that effects many types of plants including cannabis. Infected plants display white powdery spots on the leaves and stems. This fungus can infect a farmer's whole crop and destroy everything they worked for. The more we learn about this pathogen, the better chance we have for prevention. 

How its killed by things
How it spreads

"Like other fungi, mildew reproduces in two ways: Sexually, where the genetic material is recombined, and asexually, where the offspring and the mother fungus are genetically identical. The researchers now demonstrate that the success of the two reproduction methods could not be more different: “Mildew fungi detected on afflicted host plants have only successfully reproduced sexually every few centuries, primarily reproducing asexually instead,” explains Wicker."

"Powdery mildews seldom kill their host but are responsible for water and nutrient loss and impaired growth and development. They can increase respiration and transpiration, interfere with photosynthesis and reduce yields"
"Powdery mildew fungi are obligate parasites (organisms that are totally dependent on a living host for its nutrients) and all but a few are ectoparasitic (living on the outside of the host)."

The first question that people usually have is: 
Is Powdery Mildew Systemic?
Short answer: No, it is not systemic.
Long answer:

“Unlike most fungal pathogens, powdery mildew fungi tend to grow superficially, or epiphytically, on plant surfaces. During the growing season, hyphae are produced on both upper and lower leaf surfaces, although some species are restricted to one leaf surface only. Infections can also occur on stems, flowers, or fruit. Specialized absorption cells, termed haustoria, extend into the plant epidermal cells to obtain nutrition. While most powdery mildew fungi produce epiphytic mycelium, a few genera produce hyphae that are within the leaf tissue; this is known as endophytic growth.
Conidia (asexual spores) are also produced on plant surfaces during the growing season. They develop either singly or in chains on specialized hyphae called conidiophores. Conidiophores arise from the epiphytic hyphae, or in the case of endophytic hyphae, the conidiophores emerge through leaf stomata.”

An endophyte is an endosymbiont, often a bacterium or fungus, that lives within a plant for at least part of its life cycle without causing apparent disease.

Systemic– pertaining to or affecting the body as a whole.

So powdery mildew is not systemic. If it were systemic it would grow throughout the plant but it is only endophytic which means it is possible for spores to hide out in plant tissue but it won’t spread throughout the plant.

“Mildew spores are carried by the wind, air ventilation systems, clothing, pets and just about any living thing that moves about. Mildew spores will remain dormant until environmental factors offer the optimum conditions for them to come to life. Humidity levels over 55% and warm temperatures, along with over crowding are conducive to mildew growth. Once awakened, powdery mildew spores are likely to attack young plants first. It will then spread over the entire plant, infecting stems and buds, in addition to leaves.”


Powdery mildew asexual spore formation.

Powdery mildew asexual spore formation.

“Powdery mildews are polycyclic diseases that can impair photosynthesis, stunt growth, and increase the rate of senescence of host tissue.”

“At the end of the growing season, powdery mildew fungi produce sexual spores, known as ascospores, in a sac-like ascus (pl. asci) enclosed in a fruiting body called a chasmothecium (pl. chasmothecia) (cleistothecium is a former term for this structure that is still widely used). The chasmothecium is generally spherical with no natural opening; asci with ascospores are released when a crack develops in the wall of the fruiting body. This type of fruiting body is unique among the Ascomycota. A variety of appendages may occur on the surface of the chasmothecia. These appendages are thought to act like the hooks of Velcro fastener, attaching the fruiting bodies to the host, particularly to the bark of woody plants, where they overwinter.”




Types of Powdery Mildew that effect hemp
Leveillula taurica
Oidiopsis taurica [anamorph]
Sphaerotheca macularis
Sphaerotheca humuli
Oidium sp. [anamorph]

“The polycyclic disease cycle of L. taurica is similar to that of other powdery mildew species. It overwinters (as chasmothecia) in crop residues above the soil surface. Under favorable climatic conditions, the chasmothecia open and release ascospores, which are wind-dispersed. The ascospores enter the host through its stomata, germinate, and colonize the host’s tissues with its mycelia. The pathogen then begins to produce its asexual conidia, either singly or on branched conidiophores. The conidia exit through the host’s stomata and serve as a secondary inoculum to spread disease after initial infection. In the fall, the pathogen undergoes sexual reproduction and again produces chasmothecia, its dormant, overwintering structure”

Conditions Favoring Powdery Mildew

  • High relative humidity at night
  • Low relative humidity during day

“70-80F (22-27C) temperatures (These conditions prevail in spring and fall)
The spores are carried by air currents and germinate on the leaf surface. Liquid water on leaves inhibits spore germination. The fungus grows on the leaf surface but sends fine threads (haustoria) into the cells to obtain nutrients. From the time a spore germinates to the time new spores form may require only 48 hr. High humidity favors spore formation while low humidity favors spore dispersal.”


What I will do next time to avoid PM:
-I bought an inline fan to cycle my air
-I’ll buy another fan for inside my tent
-Spray with LABS, 420 bully, or H2O2
-UV light every week or so


Information about UV-C light

Ultraviolt light (UV) occurs from the sun. The wavelength of UV is outside of our visible range. The lowest wavelength colour we can see is violet, hence the name for light with a wavelength lower than this is ultraviolet.

UV ranges from 100 nanometres to 400nm. UV is further divided into UV-A, UV-B and UV-C.
UV-C: ranges from 100nm – 280nm (germicidal)
UV-B: ranges from 280nm- 315nm (sunburn)
UV-A: ranges from 315nm – 400nm (blacklight)

We here are interested in the UV-C rays, also known as „short wave ultraviolet radiation“. UV-C`s germicidal properties have been known since the 1930s. They are used in labratories in order to destroy bacteria, mould and viruses. UV-C rays are able to penetrate the outer membrane of microbes and stop them from reproducing. The specific wavelength of 253,7 nm is known to break the DNA of pathogens.

In May 2007, two Dutch inventors, Arne Aiking and Frank Verheijen were granted an International Patent on a method of treating live plants and mushrooms against pathogens with UV-C light. In the past UV-C had only been used to sterilise things like air and water. The difference with proposing to use UV-C to fight pathogens on living plants is that you shouldn`t use any more than is necessary, otherwise there is a risk to health of the plant.

Mr. Aiking and Verheijens invention is a mobile UV-C lamp that travels up and down in commercial greenhouses. The light intensity of a UV-C germicidal light bulb is usually expressed in a term called „microwatts per square centimetre“. Mr. Aiking and Verheijen suggest a range between 2,500 and 150,000 mircrowatts of UV-C energy over a given 24 hour period.

E.g. A 100 Watt UV-C lamp will output approximately 14,000 microwatts of energy over a square centimetre, in one second, from a distance of two centimetres away. If we leave it there for one minute we have to multiply that figure by 60. (840,000 microwatts). Look at the following chart:
1cm 21,000 µW/cm2 UV output
2cm 14,000 µW/cm²
3cm 10,000 µW/cm²
4cm 8,000 µW/cm²
5cm 6,700 µW/cm²

100cm 360 µW/cm²

This shows that a second`s exposure at 1cm distance is roughly equivalent to a full minute`s exposure 1 meter away (360 x 60 = 21. 600µW/cm²).

We use the UV-C light to fight against powdery mildew in our plantation. Tests have shown that powdery mildew is killed when given a dose of 1720 µW of UV-C per square cm. For exemple if I took a 100 Watt UV-C lamp and positioned it two centimetres away from the mildew, I would need to switch on the UV-C lamp for just 1/10 of a second to kill it.

Spider mites could possibly also be effectively treated with UV-C but with amounts that are hundreds of times more compared to something like powdery mildew. We therefore use it only against powdery mildew.

The safest way to use UV-C on plants appears to be regular, smaller doses rather than a single, lager hit. We therefore use the UV-C light every day for a short time. The pathogen must be hit directly with the UV-C rays in order to be affected. If it is protected by a leaf, the UV-C will not be effective, because UV-C will not penetrate through leaves.

UV-C light has an intense burning effect on human skin and can quickly damage your eyes.
Protect your eyes with special eyeglasses
Don`t expose your skin to UV-C rays
The wrong amount of UV-C will damage your plant

More Q & A:

To be effective, how close to the surface do the lamps need to be?

The exposure of germicidal ultraviolet is the product of time and intensity. High intensities for a short period and low intensities for a long period are fundamentally equal in lethal action on bacteria. The inverse square law applies to germicidal ultraviolet as it does to light: the killing power decreases as the distance from the lamps increases. The average bacterium will be killed in ten seconds at a distance of six inches from the lamp in an American Ultraviolet Germicidal Fixture.

How do germicidal lamps kill?

Ultraviolet light in the germicidal wavelength – 185-254 nanometers – renders the organisms sterile. When organisms can no longer reproduce, they die.

From my micro book

I hope that helps! Please let me know if I missed anything!

-Herbal Visionz