Management of Valsa Canker on Apple with Adjustments to Potassium Nutrition.

Valsa canker, caused by the fungus Valsa mali, is one of the most destructive diseases of apple in the primary production areas of China and other East Asian countries. Currently, there are no effective control methods for this disease. We investigated the occurrence of Valsa canker in 24 apple orchards in Shaanxi Province in concert with foliar nutrient analysis, and found that there was a significant negative correlation of leaf potassium (K) content with incidence and severity of Valsa canker. Fertilization experiments showed that increasing tree K content enhanced resistance to pathogen colonization and establishment. Apple trees with leaf K content greater than 1.30% exhibited almost complete resistance to Valsa mali. Field trials demonstrated that increasing K fertilization could significantly reduce disease incidence. Improved management of tree nutrition, especially K content, could effectively control the occurrence and development of Valsa canker.

Field Application of Calcium to Reduce Phytophthora Stem Rot of Soybean, and Calcium Distribution in Plants.

The effect of calcium compounds [Ca(HCOO)2-A and Ca(NO3)2] on the incidence of Phytophthora stem rot of soybean (Glycine max) cv. Tanbakuro was investigated in the field. Disease incidence in control plants in three fields naturally infested with Phytophthora sojae ranged from 11.7 to 52.0% at 140 days after transplanting. Independent of the pathotype diversity, 4 and 10 mM of the calcium compounds applied twice (prior to transplanting and 14 days after transplanting) significantly suppressed disease incidence and delayed onset. Ca(HCOO)2-A (Suicaru) was more effective than calcium nitrate for reducing disease incidence. In most cases, the calcium amendments increased plant height, number of nodes and pods, and seed yields, and reduced low-quality seeds. Scanning electron microscopy with fresh samples showed increased accumulation of calcium crystals around the cambium and xylem elements of soybean plants treated with 10-mM Ca(HCOO)2-A and Ca(NO3)2. Mycelial penetration was inhibited at these sites. These results indicated that calcium-rich areas may be more resistant to invasion by P. sojae, and the calcium crystals may play an important role in calcium ion storage and its availability for those tissues to maintain long-term field resistance.

Select Calcium Compounds Reduce the Severity of Phytophthora Stem Rot of Soybean.

This study investigated the effects of several calcium compounds on Phytophthora stem rot of soybean (Glycine max) and fungal growth and zoospore release of a Phytophthora sojae isolate in vitro. All concentrations of five formulated calcium products [Ca(COOH)2-A, Ca(COOH)2-B, Ca(COOH)2-C, CaSO4-A, and CaCl2-A] and two chemical compounds [CaCl2 and Ca(NO3)2] applied prior to inoculation significantly suppressed disease incidence. Among all the products and chemicals, Ca(COOH)2-A was the most effective in suppressing the incidence of disease. In most cases, no significant relationship was observed between inhibition of growth rate in vitro and disease reduction in growth chamber tests. Therefore, disease suppression recorded in laboratory experiments using pathogen mycelium was likely due to the responses of plant tissues rather than the direct inhibition of pathogen fungal growth by the calcium compounds. The extent of disease reduction was related to increased calcium uptake by plants, suggesting that calcium was the effective element in reducing Phytophthora stem rot. Seedling tray experiments using zoospores indicated that the application of 10 mM Ca(COOH)2-A was more effective for reducing incidence of disease under growth chamber conditions, compared to other concentrations. The presence of 4 to 20 mM of all seven calcium solutions decreased the release of zoospores, although 0.4 mM of all compounds significantly increased zoospore release. Therefore, disease reduction in the growth-chamber experiments was due to the multiple effects of direct suppression on zoospore release and fungal growth in combination with the response of the host plant tissue to Ca(COOH)2-A.

A Statistical Comparison of the Blossom Blight Forecasts of MARYBLYT and Cougarblight with Receiver Operating Characteristic Curve Analysis.

ABSTRACT Blossom blight forecasting is an important aspect of fire blight, caused by Erwinia amylovora, management for both apple and pear. A comparison of the forecast accuracy of two common fire blight forecasters, MARYBLYT and Cougarblight, was performed with receiver operating characteristic (ROC) curve analysis and 243 data sets. The rain threshold of Cougarblight was analyzed as a separate model termed Cougarblight and rain. Data were used as a whole and then grouped into geographic regions and cultivar susceptibilities. Frequency distributions of cases and controls, orchards or regions (depending on the data set), with and without observed disease, respectively, in all data sets overlapped. MARYBLYT, Cougarblight, and Cougarblight and rain all predicted blossom blight infection better than chance (P = 0.05). It was found that the blossom blight forecasters performed equivalently in the geographic regions of the east and west coasts of North America and moderately susceptible cultivars based on the 95% confidence intervals and pairwise contrasts of the area under the ROC curve. Significant differences (P < 0.05) between the forecasts of Cougarblight and MARYBLYT were found with pairwise contrasts in the England and very susceptible cultivar data sets. Youden's index was used to determine the optimal cutpoint of both forecasters. The greatest sensitivity and specificity for MARYBLYT coincided with the use of the highest risk threshold for predictions of infection; with Cougarblight, there was no clear single risk threshold across all data sets.

Relative Susceptibility of Selected Apple Cultivars to Colletotrichum acutatum.

Eighteen apple cultivars were tested in the field and laboratory for their relative susceptibility to one of the bitter rot pathogens, Colletotrichum acutatum. Fruit were inoculated in the field at 3 to 4 weeks preharvest with cheesecloth strips soaked in a conidia suspension. In the laboratory, detached fruit were inoculated using a conidia suspension in capped, sterile microcentrifuge tubes attached to the fruit surface with modeling clay. The same fruit as above also were inoculated over a wound on the side opposite the nonwounded inoculation. Fruit were tested for relative susceptibility to the fungus with five criteria: disease incidence and severity of attached fruit in the field, disease incidence and severity of detached fruit in laboratory inoculations of nonwounded fruit, and disease severity in laboratory inoculations of wounded fruit. Relative cultivar ranks from field tests were not reproducible in the 2 years studied, whereas laboratory tests showed moderate reproducibility with nonparametric rank correlation tests. Based on the laboratory data from 2 years of study, cultivars were classified into four relative-susceptibility groups: most susceptible: Pristine, Honeycrisp, and Ginger Gold; highly susceptible Yataka, Sansa, Arlet, and Enterprise; moderately susceptible: Sunrise, Golden Supreme, PioneerMac, GoldRush, Golden Delicious, and Creston; and least susceptible: Fuji. Compared to previous cultivar rankings, the results of the present study indicate that new apple cultivars from the first NE-183 planting show no improvement in resistance to C. acutatum.

Effects of Calcium Salts on Growth, Polygalacturonase Activity, and Infection of Peach Fruit by Monilinia fructicola.

The effects of several calcium salts on growth, polygalacturonase (PG) activity, and infection of peach fruit by Monilinia fructicola were determined. All salts except calcium formate, calcium pantothenate, and dibasic calcium phosphate reduced growth of M. fructicola on amended potato-dextrose agar (PDA) after 7 days. Minimal growth occurred on PDA amended with calcium propionate. Calcium hydroxide, calcium oxide, calcium silicate, and calcium pyrophosphate reduced growth by approximately 65% compared with the control. Substances that were inhibitory on amended PDA were also inhibitory in potato-dextrose broth (PDB) but varied in effectiveness depending on whether PDB or PDA was used. The correlation between the amount of growth on PDA versus that in PDB was not significant. Fungal PG activity was inhibited by all salts used in this study except dibasic calcium phosphate and calcium tartrate. Greatest inhibition of PG was associated with calcium propionate followed by calcium sulfate, tribasic calcium phosphate, calcium gluconate, and calcium succinate. The activity of calcium salts was not affected by pH. PG activity was correlated with growth on PDA (r s = 0.48, P ≤0.04) but was not correlated with growth in PDB. When inoculum was sprayed on detached fruit, the incidence and severity of brown rot were least on fruit that had been dipped in solutions of calcium propionate or calcium silicate. When inoculum was applied as a localized drop to wounded fruit that had been dipped in a solution containing 1,200 mg of calcium per liter, brown rot severity was least for fruit treated with calcium oxide and calcium hydroxide. For nonwounded fruit and drop inoculations, calcium hydroxide was the most effective in reducing brown rot incidence, and all salts reduced rot severity similarly. There were significant correlations between growth in PDB and disease incidence and severity 4 days after inoculation at both calcium levels (600 and 1,000 mg per liter). Furthermore, disease incidence and severity at both levels of calcium were also correlated significantly with PG activity. Future investigations on brown rot and calcium salts should utilize PDB for in vitro investigations and spray mist inoculations for fruit studies.

Detection of impervious tissue in tree bark with selective histochemistry and fluorescence microscopy.

Use of conventional histochemical tests in conjunction with fluorescence microscopy has validated the concept of impervious tissue in the bark of trees. Application of phloroglucinol + HCl or toluidine blue O selectively quenched lignin autofluorescence and allowed visualization of intracellular suberin lamellae previously undetected. Fluorescence of intracellular lamellae was quenched with Sudan black B and enhanced with Sudan IV thus providing evidence for the suberized nature of a tissue heretofore regarded as nonsuberized.