Red Rust of Guava:

Causal organism:

                  Cephaleuros virescens

Taxonomy:

                 ▪ Kingdom: Archaeplastida

                 ▪ Phylum: Chlorophyta

                 ▪ Class: Ulvophyceae 

                 ▪ Order: Trentipholiales

                 ▪ Family: Trentipholiaceae 

                 ▪ Genus: Cephaleuros

                 ▪ Species: C.virescens

Symptoms:

                 1. Orange yellow rust pustules are appear on adaxial surface of the leaves (mostly)                             

                 2. Yellow coloured pustules are due to hematochrome

             3. This disease affects both leaves and fruits:

          Leaves: 

              Small brown specks are seen in patches and raised brown algal growth also seen.

         Fruits:

                       Lesions are small, dark green to brown or black some times.

Lifecycle:

        Asexual reproduction: 

             The older algal discs produce hair-like initials at the some of the 

radiating rows of cells. 

             After rupturing of the cuticle or epidermis, it differentiate 

into sporangiophores or setae. 

             The sporangiophores terminate in a swollen cell called ‘vesicle’. 

              Usually four or eight 

or even more light orange, unicellular protrusions or sterigmata come out from the vesicle. 

              Each of them develops into an orange, oval to globose zoosporangium attached to the apex 

by a folded stalk, ‘pedicel’. 

             Under favourable conditions sporangia release 8 to 30 zoospores through an ostiole.  

             Zoospores are ovoid or spherical and biflagellate. 

            The zoospores swim actively for about 10 minutes, come 

to rest and lose their flagella. 

            Due to repeated cell wall formation in the resting zoospore, the 

supracuticular ‘primary disc’ cells penetrate the host leaf and form a ‘secondary’ disc in the 

plant tissue.

      

       Sexual reproduction: 

            The gametangia arise in the thallus as large, sessile and flask-shaped 

cells. 

            Eight to 32 but usually 16 biflagellate isogametes per gametangium are produced. 

            Copulation takes place between two isogametes either from the same gametangium or from 

different gametangia resulting in the formation of zygote. 

            The zygote produces a dwarf 

sporophyte, bearing a small dehiscent microsporangium. 

            After meiosis each microsporangium 

releases four quadriflagellate ‘microzoospores’. 

            These microzoospores infect the host very rarely.

 Mode of spread: 

            Zoosporangia are disseminated by wind and rain splashes.