Pathogen Propagules Research Articles

Rice disease situation in mangrove and associated swamps in Sierra Leone

Abstract The importance of mangrove swamp rice cultivation in Sierra Leone and in the West African subregion in general is defined and diseases occurring are listed and discussed including some indication of their economic importance. Generally it was observed that disease severity was less pronounced under this cultivation than in other types of rice growing areas. Several factors were considered responsible for this phenomenon, namely: the non‐use of nitrogenous fertilizers in mangrove swamp rice fields by the farmers; the breezy nature of the swamp environment which leads to faster drying up of water drops on the rice plant thus inhibiting the infection process; and the low pH and salinity, which, particularly in the off‐season, is thought to reduce the number of viable propagules of several rice pathogens. Rokupr was found to be suitable for the screening of rice varieties resistant to rice blast (Magnaporthe grisea (Hebert) M. E. Barr) due to the presence of a rich flora of races of the fungus. A str...

Stability in an insect-pathogen model incorporating age-dependent immunity and seasonal host reproduction

A modified SIRS model is developed as a framework for the study of epizootiological dynamics in an insect-pathogen system. Linearized stability analysis reveals that the system with one immune and one susceptible host class can exhibit stable, periodic or unstable behavior depending on model parameters. In general, high pathogenicity, short pathogen propagule lifespan and high host reproductive rate are stabilizing influences. Pathogen transmissibility and propagule production/host do not influence local stability. The effect of seasonal host reproduction is studied because most insect hosts are seasonal in temperate climates. The basic stability dependence on model parameters holds except as modified by the length of the reproduction interval. The results of this study are compared with the recent work of Anderson and May.

Persistence of Phoma exigua xar. foveata and Polyscytalum pustulans in dry soils from potato stores in relation to reinfection of stocks derived from stem cuttings

SUMMARYThe incidence of Phoma exigua var. foveata and Polyscytalum pustulans in dry soil and dust from potato stores was assessed at 10 farms in Scotland producing virus‐tested stocks of seed potatoes derived from stem cuttings (VTSC). Samples were collected on three occasions during 1976: during the storage period (February‐March), soon after the potatoes were planted (May) and just before the new crop was lifted and stored (August‐October). Both pathogens were detected at all three sampling times but P. exigua var. foveata was more frequently detected at the last one than P. pustulans. Soil and dust from various sites within the stores, such as floors, ledges, graders and boxes, were contaminated by these pathogens. Propagules of both pathogens remained airborne for at least 12–17 min after the floors were swept.P. exigua var. foveata remained viable for at least 16 months in dry soil from tuber surfaces stored at 4–6°C. When VTSC tubers were dusted before planting with a dry store soil contaminated by P. exigua var. foveata the gangrene potential of the progeny tubers was much greater than that from untreated tubers or from tubers treated with an uncontaminated soil. The role of inoculum surviving in dry soil is discussed in relation to reinfection of VTSC stocks.

GENETICS OF DISEASE RESISTANCE IN PLANTS

Most plant diseases are infectious. They result from the interaction of two organisms, the host plant which is the suscept of the disease and the patho­ gen which is the causal agent of the disease. A wide array of pathogens cause disease. These include many fungi, bacteria, viruses, mycoplasma, nematodes, and parasitic phanerogams. Several abiotic environmental factors can also cause plant disease. Disease resistance may be partial or nearly complete. Complete susceptibility of a plant to a pathogen is rare. In such hosts, nothing impedes pathogen development throughout the disease cycle. A large proportion of pathogen propagules reach the host and infect it. Pathogen development is rapid and abundant in the host. The pathogen reproduces rapidly and over a prolonged period. If the pathogen has an efficient means of dispersal, it spreads rapidly to new infection courts. Under favorable en­ vironmental conditions an explosive epiphytotic results. In the resistant plant, host and pathogen are in conflict; pathogen growth and development are suboptimal. The resistance can take various forms and numerous genetic systems are known to result in host resistance. Genetic variation within the pathogen is of extreme importance in disease resistance studies and in disease control by means of host resistance (1-4). Some forms of host resistance function against certain races or biotypes of a pathogen but not all against others. In contrast, other forms of resistance function against all biotypes of a pathogen.