Rice leaf blast, caused by Pyricularia grisea, usually starts in a region with a general leaf blast epidemic. Most (if not all) inoculum sources are believed to come from infected seedlings which are left in mats in the fields and used to fill vacant space in rows after mechanical transplanting. We determined the conditions that promote leaf blast within a seedling mat and the threshold level of disease severity in the mat that leads to general epidemics in a region. When the mat contained a few seedlings infected with the blast fungus, waves of newly infected seedlings appeared at several-day intervals across a mat. Each wave was caused when favorable conditions for infection occurred; this was determined by the length of the leaf wetness period and the temperature during the period. The leaf wetness period for infection began when it rained and continued until total cumulative solar radiation had reached 5.9MJ/m2. The criterion to judge whether or not an infection occurred during a leaf wetness period was based on the ratio of the actual percentage of fungal penetration for a given leaf wetness period and the potential percentage of fungal penetration over a long enough time at a given temperature. When this ratio exceeded 0.1, the conditions were favorable for infection. Diseased mats can play a role as a potential inoculum source for general epidemics of leaf blast, when the disease severity within the mat is greater than a certain threshold, which in this study was when 90cm2 of the leaves in a mat were dead.
Glucose, , sucrose, sodium tartrate, or L-arginine were tested as a carbon source in eight basal media using 99 bacterial strains belonging to 63 species in 14 genera, including some strains of non-plant pathogens. From these results and those from additional tests using nine carbon sources, Dye's medium OY was useful for testing carbohydrates and sugar alcohols, and BMC developed in this study as a basal medium was useful for testing various carbon sources such as carbohydrates, sugar alcohols, organic acids and amino acids. BMC medium contains 0.4g NH4H2PO4, 0.6g (NH4)2HPO4, 0.2g KCl, 0.2g MgSO4⋅7H2O, 5g NaCl, 0.2g yeast extract, 10ml 0.5% bromthymol blue (BTB) solution, and 16g agar in 990ml of distilled water with pH 6.8. This medium can be used only for bacteria giving a positive reaction within 1 week on BMC supplemented with 0.5% glucose. Further study is needed to develop a medium for bacteria that require nutritionally rich media, such as the genus Clavibacter. When the results obtained by different basal media are examined, attention must be paid to the nutrient levels in the media and to the culture durations. It is especially important to note that negative results may be questionable because of an absence or excess of certain nutrients.
Protoplasts were isolated from the parent isolate (SR8501) of Sclerotium rolfsii by a previously reported combination of enzymatic methods using Cellulase Y-C (1%), Driselase (1%), Zymolyase-20T (0.1%), Chitinase (0.1%) and Pectolyase Y-23 (0.1%). Most of the protoplasts had a single nucleus. A single protoplast suspended in a small drop of potato broth containing 0.6M sucrose as an osmotic stabilizer was able to regenerate its cell wall and revert to mycelium with a relatively high frequency. Virulence on soybean susceptible to the parent isolate, pathogenicity to some other crops, and cultural characteristics were altered in many of the revetant isolates, compared with the parent isolate. These characteristics of the revertant isolates were stable in their frequent transfer to new media. This variability may essentially reflect the genetic potential of the parent isolate, although a mutation might be involved in some of the new characters.
In 1999 and 2000, powdery mildew was found on leaves, stems, peduncles and flower buds of chocolate cosmos (Cosmos atrosanguineus (Hook.) Voss) in Tokyo, Japan. The causal agent was identified as Sphaerotheca fusca Blumer emend Braun.