Brown leaf spot disease and fertilizer interaction in irrigated rice growing on different soil types
Studies have been undertaken in a 1000 ha area of irrigated rice (Oryzica sativa) at Caroni (1975) Limited to determine the effect of the fertilizer programme on the incidence of important diseases. Over a period of three years higher levels of brown leaf spot (Cochliobolus miyabeanus) on rice varieties Oryzica 1 and Oryzica 5 on three different soil types were associated with increasing levels of leaf P, from a low of 0.149% of dry matter (DM) to a high of 0.396% DM. On the Washington silty clay loam series (Inceptisol) brown leaf spot incidence was lowest when leaf P was between 0.135% and 0.149% of DM. However, disease incidence was higher when leaf P levels fell to 0.133% of DM or rose above 0.149%, under conditions where N was more than adequate. The moderate levels of the disease experienced over the period had no effect on yield, as grain infection was minimal. The results support the conclusion that the incidence of brown leaf spot on irrigated rice at Caroni is influenced by sub-optimal levels paticularly of P. Careful monitoring and managememt of P nutrition is seen as an important part in the overall strategy for controlling the disease.
Genetic Diversity and Aggressiveness of Bipolaris oryzae in North-Central Thailand
One hundred and ten isolates of Bipolaris oryzae, the causal agent of rice brown spot disease were collected from paddy fields in four provinces of north-central Thailand, including Ang Thong, Chai Nat, Lop Buri, and Sing Buri. DNA polymorphism of some Bipolaris oryzae isolates was determined by VNTR, ISSR and RAPD markers. Only VNTR-MR primer showed different fingerprint patterns among these isolates, therefore this primer was selected to study genetic diversity of the Bipolaris oryzae population. In total, there were three haplotypes corresponding to the results from cluster analysis; each of the three clusters shared identical haplotype. The majority of the isolates were separated into group A (88.18%), indicating predominant asexual reproduction of clonal population. However, there was no relationship between haplotype and either collection provinces or aggressiveness on rice. Among four rice varieties tested, including Khao Dawk Mali 105 (KDML 105), RD31, Pathum Thani 1, and Jao Hom Nin (JHN), JHN was the most resistant variety, while KDML 105 was the most susceptible to brown spot disease.
Germination of Botrytis cinerea conidia in vitro in relation to nutrient conditions on leaf surfaces
Conidia of different isolates of Botrytis cinerea Pers. ex Fr. varied widely in ability to germinate in water. Sugars and amino acids were added at concentrations similar to natural levels on leaf surfaces. To a limited extent simple sugars promoted germination. Fructose was most effective whilst sucrose was least effective. Amino acids stimulated germination with the exception of taurine and lysine. Lysine reduced the stimulatory effect of a mixture of amino acids. Germination of conidia was only increased by an inorganic nitrogen source, ammonium salts or by a mixture of mineral salts, when glucose was present. Growth factors, yeast extract, gibberellic acid and kinetin had no effect on germination but indole-acetic acid caused a small stimulation. Abscisic acid depressed germination. It is suggested that, like soil fungi, the germination of spores of phylloplane fungi may be nutrient-dependent, or independent with respect to exogenous nutrients. Poor in vivo germination of B. cinerea may be associated with competition.