- Author: Steven T. Koike
White mold disease, caused by the fungus Sclerotinia sclerotiorum, is causing damage to a number of vegetable crops in California and Arizona during the late 2010 and early 2011 months. On the coast of California, white mold is being found on crucifer crops such as broccoli and cauliflower. In the desert regions white mold is causing damage on broccoli, cauliflower, celery, lettuce, and other vegetables (for lettuce this disease is commonly called lettuce drop). White mold incidence on these crops appears to be greater than normally observed. See photos 1 through 6 below.
The first symptoms on most vegetable crop hosts are small, irregularly shaped, water-soaked areas on stems, leaves, pods, or flower heads. These infections quickly develop into soft, watery, pale brown to gray rots. Rotted areas can expand rapidly and affect a large portion of the plant. Diseased tissues eventually are covered with white mycelium, white mycelial mounds that are immature sclerotia, and finally mature, hard, black sclerotia. Mature sclerotia usually form after tissues are rotting and breaking down. Plants with infections on the main stems can completely collapse and fall over.
The black sclerotium is the survival stage of the fungus and can measure from ¼ to ½ inch long. Sclerotia are found in the soil and can directly infect plants if stems are in close proximity. However, these winter cases of white mold are due to ascospore infections. If sufficient soil moisture is present, shallowly buried sclerotia germinate and form small, tan mushroom-like structures called apothecia (photos 7 and 8). Ascospores (photos 8 and 9) are released from apothecia and carried by winds to the host plant. These ascospores are responsible for these winter infections and result in disease of the above-ground parts of plants. The relatively cool, moist weather found in most regions has allowed for the production of apothecia production and ascospore releases.
For ascospores to start colonizing plant tissues, nutrients and plant fluids from damaged tissues are usually needed. This is why white mold is very severe if ascospores land on compromised tissues such as lettuce leaves with tip burn, leaves and heads damaged by frost or other factors, stems with open wounds or exposed leaf traces (vascular tissue in the stem that is left exposed when a lower leaf falls off), and senescent leaves and stems.
Controlling white mold under these winter weather conditions is difficult. Protective fungicides provide some assistance and can be used effectively in lettuce. However, such fungicides need to be applied prior to ascospore flights and usually will require multiple sprays. Fungicides may not be warranted for crucifer crops.
Steve Koike thanks Jeff Rollins and Karen Chamusco for assistance with photographs for this article.
Photo 1: White mold (lettuce drop) on romaine lettuce.
Photo 2: White mold (lettuce drop) on romaine lettuce, showing white mycelium and two black sclerotia.
Photo 3: White mold on broccoli stems.
Photo 4: White mold on broccoli stem, showing white mycelium and one black sclerotium (center).
Photo 5: White mold on cauliflower head, showing white mycelium.
Photo 6:White mold on celery, showing numerous black sclerotia.
Photo 7: One sclerotium and several apothecia (spore producing structures) of Sclerotinia sclerotiorum.
Photo 8: Microscopic view of the spore-producing apothecium of Sclerotinia sclerotiorum. Note the lined-up ascospores (red) ready to be released. Photo used by permission (K. Chamusco).
Photo 9: Microscopic view of ascospores lined-up in a tube (called an ascus) and ready to be released. Photo used by permission (J. Rollins).
- Author: Steven T. Koike
Experienced growers, pest control advisors, and other field professionals involved with broccoli already know that the winter period can signal increased problems due to head rot (also known as pin rot). Favored by cool temperatures and prolonged periods of moisture from rain, dew, and fog, broccoli head rot continues to be a damaging and yield-reducing factor because preventative measures have yet to be consistently effective. Spray applications have not proven to be consistently reliable tools to prevent head rot. While some cultivars (especially those having rounded, dome-shaped heads) may be less susceptible to head rot, true resistance has not been demonstrated for cultivars grown in California. Trying to avoid the use of overhead sprinkler irrigation is apparently the only cultural practice that helps reduce disease; however, the winter and early spring weather will enhance head rot even if growers use drip or furrow irrigation.
Field personnel should remember that two types of head rot affect the crop in California. For bacterial head rot, initial symptoms on the immature broccoli heads consist of a water-soaked or greasy discoloration of the surfaces of small groups of the unopened flowers. Later, the affected portions of the head turn brown to black and the infection spreads and affects larger parts of the head. The tissue becomes soft and gives off a very bad odor. For bacterial head rot there will not be any fungal growth unless secondary molds colonize and cause further decay.
The second type of head rot is Alternaria head rot. For this fungal problem, early symptoms consist of a water-soaked discoloration that later turns dark brown to black. Tissues infected with Alternaria are usually not as soft and smelly as heads infected with the bacterial pathogens. Alternaria readily produces dark green spores on the diseased head tissue. Secondary molds and bacteria cause further decay.
Photo 1: Bacterial head rot of broccoli
Photo 2: Bacterial head rot of broccoli
Photo 3: Alternaria head rot of broccoli
Photo 4: Alternaria head rot of broccoli