Abstract
The present paper reports the mode of infection of rice leaves by the Fusarium leaf spot fungus, especially the relation between conidial fusion and invasion behaviour.
Many of the conidia inoculated on the surface of rice leaves fused with each other by means of germtubes or projections and growth of many vigorous hyphae occurred before invasion. Invasion into rice leaves mostly took place by the vigorous superficial hyphae.
In order to observe the details of the conidial behaviour before invasion into leaves, epidermal strips of onion bulb scales, which were floated on water in petri dish after immersing in alcohol and washing in water, were used. Inoculated conidia fused with each other and formed a conidial complex. The hyphae growing out from the conidial complex were vigorous while the hyphae or germtubes from single conidia, which were small in size, were short and slender. Penetration into the onion epidermal cells mostly took place by the vigorous superficial hyphae. Some of the conidia forming a conidial complex lost the contents entirely or partially. It appears that the contents of some conidia is translocated through the connecting hyphae to some other conidia, from which vigorous hyphae develop.
The observations using onion epidermal strips suggest that a conidial complex acts as a unit in invasion behaviour, and vigorous hyphae on rice leaves grow utilizing the contents of conidia in the complex. Namely, the conidial fusion may be necessary for the fungus to invade into the leaves.
The vigorous hyphae from the conidial complex, coming in contact with stomata, formed appressorium-like structures with various sizes. From the structures infection hyphae developed passing through stomatal slits and swelling in substomatal cavities. The substomatal hyphae grew profusely into the intercellular spaces and then entered mesophyll cells. The development of hyphae in vascular bundles and epidermal cells was poor. About three days after inoculation short branched conidiophores grew out through stomatal slits to form spore lumps.
