(a) Describe the causal organisms, disease cycle and control measures of Early Blight of Potato and Blast of Rice. 10+10=20 (b)…

(a) Early Blight of Potato and Blast of Rice

Early Blight of Potato is caused by the fungus Alternaria solani. The causal organism produces conidiophores that are short, simple (unbranched), septate, and dark brown, bearing obclavate, muriform conidia (with transverse and longitudinal septa) that taper into a long beak. The disease cycle begins with primary infection from infected seed tubers or crop debris overwintering in the soil. Under favorable conditions (15–25°C and high humidity), spores germinate and infect leaves via stomata or wounds. Secondary infection occurs rapidly through wind-borne conidia, leading to characteristic concentric ringed lesions ("target spots") on leaves and tubers. Control measures include planting certified disease-free seed tubers, crop rotation with non-hosts, and removal of infected debris. Chemical control involves the application of mancozeb or chlorothalonil at 7–10 day intervals. No potato cultivar is fully resistant; growing regionally recommended tolerant cultivars with balanced nutrition (plant stress favours the disease) reduces losses.

Blast of Rice is caused by the fungus Magnaporthe oryzae (syn. Pyricularia oryzae). The pathogen produces pyriform, three-celled (two-septate) conidia on conidiophores. Infection requires the formation of appressoria, which penetrate the rice cuticle. The disease cycle involves primary infection from overwintering mycelium in stubble or seed, followed by secondary spread via wind-dispersed conidia. High humidity, moderate temperatures (20–30°C), and excess nitrogen application favor disease development. Symptoms include diamond-shaped lesions on leaves, neck blast causing head drop, and node blast leading to lodging. Control strategies emphasize the use of blast-resistant varieties matched to local pathogen races (e.g., Pusa Basmati 1637, which carries the Pi9 gene), avoiding excess nitrogen, and maintaining proper water management. Fungicides such as tricyclazole, isoprothiolane, azoxystrobin or carbendazim are sprayed at the tillering and booting/heading stages; seed treatment with carbendazim or tricyclazole checks seed-borne inoculum. Biological control using Trichoderma species can also be integrated into management.

(b) Eusporangiate and Leptosporangiate Sporangia

Eusporangiate sporangia develop from a group of superficial cells (initials) on the surface of the sporophyll. They are characterized by a thick, multi-layered wall and produce a large number of spores. The annulus, a row of specialized cells involved in sporangial dehiscence, is typically absent or rudimentary. Examples include Ophioglossum, Angiopteris, Marattia and Danaea. In contrast, leptosporangiate sporangia arise from a single superficial cell (the sporogenous initial). They have a thin, single-layered wall and produce a limited, often fixed number of spores (typically 64 or fewer). A distinct annulus is present, facilitating explosive spore release. This type is characteristic of most ferns, such as Dryopteris and Polypodium.

Based on the sequence of sporangial development, fern sori are classified (Bower) as follows:

1. Simple Sori: All sporangia of a sorus arise and mature simultaneously, e.g., Gleichenia, Matonia, and the eusporangiate Marattia and Danaea (whose sporangia are fused into synangia). Osmunda is not an example: its sporangia are not grouped into sori.
2. Gradate Sori: Sporangia develop in basipetal succession on an elongated receptacle, the oldest at the apex and the youngest at the base, e.g., Hymenophyllum, Trichomanes, Cyathea.
3. Mixed Sori: Sporangia of different ages are intermingled without definite order, e.g., Dryopteris (reniform indusium), Polypodium (nude sori) and Asplenium (linear sori with a lateral indusium).

**(c) Structural Variation in Megasporophylls of *Cycas***

The megasporophylls of Cycas species exhibit significant structural variation. Each megasporophyll has a basal petiole, a middle portion bearing ovules in two lateral rows along its margins (not on the abaxial surface of a blade), and an upper sterile lamina. In C. revoluta the megasporophyll is densely tomentose and its lamina is broad and pinnately dissected; C. pectinata and C. siamensis also have pinnately divided laminae. In C. circinalis and C. rumphii the lamina is not pinnate but a reduced blade with serrate (toothed) margins and a tapering acute apex. The number of ovules ranges from about 2 to 12 per megasporophyll depending on the species, e.g., usually only two in C. siamensis.

**Primitive Features in *Cycas*** Cycas retains several primitive features linking it to ferns and early seed plants. These include:

1. Motile Sperms: The male gametes are flagellated and require water for fertilization, a characteristic of bryophytes and pteridophytes.
2. Large Ovules and No Female Cone: The ovules are among the largest in living plants, with a massive nucellus and large archegonia; the female plant bears no compact cone, as leaf-like megasporophylls are loosely arranged in a crown and the stem apex grows on through them.
3. Haustorial Pollen Tube: A pollen tube is formed in all Cycas species, but it is haustorial (absorbs food from the nucellus) rather than a sperm carrier; it bursts to release motile sperms into the archegonial chamber (zooidogamy).
4. Leaf Morphology: The foliage leaves are large, pinnate, and resemble fern fronds, showing a fern-like morphology.
5. Circinate Vernation: Young foliage leaves exhibit circinate vernation (coiled tip), a feature common in ferns. Note that megasporophylls do not show circinate vernation; this feature is restricted to the young foliage leaves.