- Author: Mark Bolda
Here is another example of orange rust, which are caused by two fungi, Arthuriomyces and Gymnoconia, the two of which are distinguished by the shape of their spores and life cycle length. To re-iterate, growth of orange rust is strictly on blackberry and is favored by cooler temperatures and high humidity, but I have yet to see an infection of this rust explode across the landscape no matter what the conditions are. Instead, several plants around a single locus will show infection at one time and the advance of the disease, if not addressed, is measured in years rather than days or weeks.
Orange rust is distinguished by the intensity of its orange color and the sharp outlines of the infected area. Usually one can see the spores by rubbing or brushing up against the leaves.
Probably the most important point about managing orange rust in the field is that it is systemic and cannot be controlled by any fungicide we have labeled in blackberries. Leaving plants alone or trying to spray one’s way out of it will end in tears. Orange rust MUST be removed by hand, that is to say the whole infected plant dug out with a shovel. It is best to remove plants directly adjacent to the infected area as well, since they are quite likely infected but not yet showing the symptoms. Take care to remove the whole plant, including the roots down to at least 12 inches. Plants should be bagged at the point of removal and disposed of properly. Personnel removing the plants should use clothing which can be disposed of or washed quite soon after working with the infected plants so as not to continue to spread spores across the blackberry field over the course of the remaining workday.
One last point, graphically illustrated in the last picture, is that groundsel in blackberry fields gets a type of rust also, but this is not the same rust the orange rust occurring on blackberry and is rather a species of Puccinia. Rusts tend to pretty specific to their hosts, so removing groundsel with the goal of reducing the rust pressure on blackberries will not work.
Yet another hat tip to the PCA and grower for letting me know about this one.
- Author: Mark Bolda
by Steven Koike, UCCE
Leaf blotch disease of strawberry is being detected in some fields in coastal California this winter. Dependent on splashing water, this disease rarely occurs unless overhead sprinklers are used or rains occur. Symptoms generally consist of tan to gray leaf lesions that commonly (though not exclusively) develop along the margin or edge of the leaflets. Leaf blotches are irregular in shape and may be surrounded by a purple border. These affected areas tend to grow fairly large; they can expand and cover from 1/4 to 1/2 of the leaflet surface.
A key diagnostic feature of leaf blotch is the presence of tiny, brown to black, fungal fruiting bodies in the gray blotches. Brown to black petiole lesions can also occur. In past seasons leaf blotch has been misdiagnosed as spray burn or phytotoxicity from applied chemicals. If uncertain about the nature of such leaf symptoms, submit samples to the UC Cooperative Extension offices in the coastal counties.
Leaf blotch disease is caused by the fungus Gnomonia comari, which also has the name Zythia fragariae. Zythia fragariae survives on strawberry crop residues. The fungus does not appear to be a true soilborne fungus, so it will not likely persist in soil unless strawberry crop debris is present. The Zythia form is especially dependent on splashing water for spore dispersal and infection. This accounts for the typical appearance of leaf blotch during winter and early spring seasons when there are rains. Leaf blotch is usually considered a minor problem and fungicide treatment programs have not been developed nor are they generally recommended. The strawberry plants usually grow out of the problem. Leaf blotch spread and development is dependent on rains and splashing water, so once the winter rains cease, leaf blotch usually becomes a non-issue for growers.