Calag Archive

Fireblight, caused by the bacterium Erwinia amylovora, was more severe in Sacramento Valley pear orchards during 1970 than any time in the previous 20 years. Following the warm winter of 1969-70, the regular pear bloom was prolonged for approximately a month with first bloom March 16 and petal fall ending in mid-April. “Rat-tail” (late season) bloom continued for several weeks during May and June. During much of this period, weather conditions favoring blight were ideal with temperatures over 65°F, and frequent rains. First widespread fireblight infections were observed in the Sacramento Valley between April 18-21, continuing through May and June.

Asevere outbreak of fireblight occurred in Bartlett pear orchards in the northern Sacramento Valley during 1971. It developed in orchards where growers had followed a thorough blight control program using streptomycin sprays (12 to 18 applications per season) exclusively during 1970 and 1971. Blight samples from these orchards were collected and checked for streptomycin resistance in May 1971. The pathogen in many of these samples was shown to be highly resistant to streptomycin. This newly developed resistance to streptomycin in the Sacramento Valley apparently accounted for the failure to control blight in 1971 with streptomycin programs that had performed successfully in 1970.

Russet (small, corky, brown spots on ¹ the surface of fruit) makes pears less attractive to buyers and so reduces prices paid for fresh fruit even though it does not affect the eating or keeping quality. Several studies have shown that sprays, dew or rain on the surface of young fruit increase russet. For this reason, fireblight control sprays applied to pear trees with young russet-susceptible fruit were evaluated for their effect on fruit russet in 1971 and 1972.

This report evaluates a method used to determine the uniformity of spray coverage in a pear fireblight control experiment in 1972. Trees used for the experiment were mature 14-year-olds in a hedgerow planting at 11 hy 22 ft spacing. The trees were uniform in size, vigorous in growth (many 5-ft shoots per season) and completely grown together in the hedge. Standard vase-shaped pruning was practiced, giving a diameter of approximately 14 ft, with a height of 15 ft after dormant pruning.

Damping-off of cotton seedlings (caused by the fungus Rhizoctonia solara Kuehn) can be a serious seedling disease in the interior valleys of southern California. Pythium spp. may cause some seed rot and seedling decline. Mercury fungicides were recently banned from use as cotton seed treatments and trials were initiated in the spring of 1970 to find effective replacements.

Even though organic wastes have been used as sources of nutrient elements for many centuries, a rational basis for their use has never been developed. Recommended rates have been based on experience and research planned without the ability to match application rates to the needs of crop plants, and with little information on the rate of biological decay of the organic materials.

Soil analysis, especially nitric acid extraction of potassium appears to be a very useful means of diagnosing potassium-deficient soils in California. Studies reported here compared two analytical methods for determining soil potassium levels. Plant growth responses were determined by greenhouse pot tests.