Clear Lake Plant Diseases

I’m afraid my Plant Diseases class from Merritt College in Oakland has despoiled me – I am always on the lookout for plant diseases! End-of-summer is a great time to spot them, as many plants have spent their reserves on reproduction, and are more vulnerable to attack. Here are some of the diseases I spotted in Clear Lake; oaks seem especially hard hit.



Sudden Oak Death –  I wasn't close enough for direct inspection, but this looks like SOD from afar; caused by the water mold, Phytophthora ramorum. 

Oak Gall – these little "nipples" are galls caused by asexual reproduction by the oak gall wasp.
When the wasp lays its eggs, hormones kick off a type of genetic engineering to create the perfect nursery (the gall).

Oak Apple Gall – this "strange fruit" indicates sexual reproduction by the oak gall wasp.

Tent Caterpillar – we found these gauzy structures all over foliage.
The residents had already moved on.

In many cases, these diseases do not cause permanent damage. An organism is simply taking advantage of a situation, and working through its own life cycle. After the class, seeing plant disease is not necessarily alarming. Plants in nature are rarely perfect; instead, they are teaming with all sorts of life forms. Learning to enjoy the whole ecosystem of a plant, both good and bad, can add to the enjoyment of nature as well as the garden. And raising your tolerance for imperfection can lower the need to combat the imperfect with pesticides.

Crown Gall

Crown gall is another bacterial disease common in the San Francisco Bay Area. Crown gall is caused by the Agrobacterium tumefaciens bacteria, and is named because masses of galls grow on the roots or root crown of infected plants. Crown gall has a wide host range including 93 families of pome and stone fruits, nuts, ornamental plants, and vines and canes. It was first studied in grape vines in Italy by Fridiano Cavara in 1897. Lack of vigor and poor growth above ground, may indicate a plant is infested with crown gall below ground. You can dig carefully around the root crown to look for evidence.

The A. tumefaciens bacteria can live in soil as a saprophyte (living on nonorganic matter) for a short time, and then invades a host plant through a wound. The bacterium transfers a piece of its plasmid DNA into a plant cell; the genetic code integrates with the plant cell’s genome. Genes on the tumor-inducing (Ti) plasmid cause the cell to divide over and over, forming a tumor. The tumor, or gall, provides shelter and a food source for the bacteria, which live and multiply between the cells of the gall. When the gall deteriorates or breaks off in the soil, the bacteria are released back into the soil. Galls typically form on plant roots, but may grow above ground on trunks and stems.

Bacterial gall growing on root

Crop rotation is an important management tool to break the disease cycle. Other measures include planting crown gall resistant plants, treating the roots of transplants with A. radiobacter (an antagonistic strain), and removing infected plants. Some experts recommend disinfecting tools used for pruning and grafting, but others dispute its effectiveness. To learn more, about crown gall, visit the IPM website: http://www.ipm.ucdavis.edu/PMG/GARDEN/PLANTS/DISEASES/crowngall.html and http://www.ipm.ucdavis.edu/PMG/GARDEN/FLOWERS/DISEASE/crowngall.html.

I was fascinated to learn that this mechanism of using the Ti plasmid of A. tumefaciens to introduce genetic code into a plant’s genome is used to introduce desirable genes into plants and provides the foundation for genetic engineering. This method is used to breed plant strains that are resistant to specific diseases, accelerating the process of selective breeding that humans have been practicing since the beginning of cultivation. This is controversial for many, but lifesaving in areas of the world, like Africa, where plant diseases devastate food crops.