What is causing the blistering on my leaves?

Edema (oedema) and intumescence are physiological disorders that cause outward blistering and bumpy growth to form on leaves, petioles, and sometimes stems. Recent weather conditions in Michigan are suspected to have caused an increase in reports of edema. Michigan had a week of sunny, warm, spring-like weather followed by a cold snap where highs were in the low 40’s and much of western Michigan experienced record snowfalls (2-8”). At this point in the season (Week 16) most greenhouses were full and some growers were watering more heavily in the sunny week prior to the cloudy, cold weekend.

by Heidi Lindberg - wollaege@anr.msu.edu, Jan Byrne - byrnejm@msu.edu, and Erik Runkle - runkleer@msu.edu

Edema is thought to be related to an imbalance of water, where plants uptake water more quickly than tissues can transpire it, causing cells to burst. It is typically observed on the lower side of leaves, usually on the tissue between the veins of the leaf. Intumescence is more related to low UV radiation inside greenhouses, and develops on the top and/or bottom of the leaves and often along leaf veins. Both are physiological disorders and do not ‘spread’ over time.

Edema on the underside of a geranium leaf; the blisters between veins. Photo: Sandra Jensen, Cornell University, Bugwood.org.

Intumescences present along the veins of the leaves of sweet potato vine. Photos: Anonymous.

Albugo spp.
Blisters or pustules on foliage of some crops can be a symptom of disease. Albugospp. infects foliage causing what is commonly referred to as white blister rust. This is a bit of a misnomer, the pathogen is not a true rust fungus, but instead is more closely related to the downy mildew pathogens. There are several different species of Albugo that occur in the U.S., each species has a specific host range. For example, one species infects Cleome, while another infects several different Brassica species. Some susceptible greenhouse grown include Amaranthus, Artemisia(wormwood), Iberis, Ipomea (sweet potato vine), Lobularia (sweet alyssum), and Portulaca. Perhaps the most commonly infected plants are cruciferous vegetables.

Intumescences present along the veins of the leaves of sweet potato vine and tomato. Photo: Heidi Lindberg.

Albugo spp. and downy mildew pathogens have some similarities in their life cycles. They are obligate pathogens, meaning that they require live host tissue for growth. They also greatly depend on water; these pathogens produce zoospores which are capable of swimming through free standing moisture (including on the leaf surface). Cool wet conditions are reported to promote infection and subsequent disease development. Albugo produces mycelium which grows within the plant tissue and forms raised wart-like growths. Sporangia, which are reproductive structures of the pathogen, are formed inside these growths.

Blistering on the stem of a tomato. Photo: Heidi Lindberg.

How can you verify which issue you have?
Knowing your host is a good starting point. For example, ivy geranium has a tendency to develop edema but is not a host for the pathogens that cause white blister rust. Ipomea (sweet potato vine) is susceptible to both edema and Albugo.

Albugo on spinach (Spinaciaoleracea). Photo: Paul Bachi, University of Kentucky Research and Education Center, Bugwood.org

White wart-like growths on the undersurface of Cleome foliage, caused by Albugo spp. Photo: Jan Byrne, MSU Diagnostic Services

The structures formed by Albugo and other white blister rust pathogens are small, in most cases they cannot be seen with the naked eye. When in doubt consider sending a plant sample to diagnostic lab. In the lab, cross sections of tissue can be viewed under a microscope to look for sporangia and other structures formed by Albugo and similar pathogens.

While there may be differences between edema or intumescences technically, the practical management strategies are similar. Both edema and intumescences are physiological disorders and do not ‘spread’ from one plant to another. Management strategies include:

  1. reducing humidity in the greenhouse by increasing ventilation and/or operating horizontal air flow fans
  2. watering highly-susceptible crops (such as ivy geranium, sweet potato vine, and tomato) carefully by not underwatering to cause plant stress, or overwatering to the point of water-logging the growing media
  3. increasing the plant spacing to improve air flow, and
  4. grouping crops that have similar watering needs to help avoid watering extremes.

However, if the environmental conditions do improve (higher light levels, more air flow, more venting), and the new growth on the plants develops blisters, it could indicate the presence of a pathogen. To try to prevent or minimize oedema/intumescence from occurring in the future, avoid having a wet growing substrate if conditions are expected to rapidly change from warm and sunny conditions to cold and cloudy conditions.

For more information, check out e-GRO Edible Alert 4.05 “Edema of Greenhouse Tomatoes,” Alert 8.16, “Intumescence on Black-eyed Susan vine,” and Alert 6.12 “Ornamental Sweetpotato Intumescence: A Physiological Disorder.”

Source: e-GRO

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