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

Part (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 branched, septate, and dark brown, bearing obpyriform conidia with transverse and longitudinal septa. 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. Resistant varieties such as Kufri Jyoti and Kufri Bahar are recommended for cultivation.

Blast of Rice is caused by the fungus Magnaporthe oryzae (syn. Pyricularia oryzae). The pathogen produces pyriform, two-celled 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 resistant varieties like BRRI dhan47 or IR64 (depending on local pathogen races), avoiding excess nitrogen, and maintaining proper water management. Fungicides such as carbendazim or isoprothiolane are applied at the tillering and booting stages. Biological control using Trichoderma species can also be integrated into management.

Part (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 the sporangia of 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 number of spores. A distinct annulus is present, facilitating explosive spore release. This type is characteristic of most ferns, such as Dryopteris and Polypodium.

Based on the mode of sporangia development, sori in ferns are classified as follows:

1. Eusporangiate Sori: These are typically coenosori (clusters of sporangia) or synangia (fused sporangia). In Marattia, the sori are large, coenosori-like structures located on the abaxial surface of the pinnae. In Danaea, the sporangia are arranged in synangia.
2. Leptosporangiate Sori: These are further categorized based on the presence of an indusium.

• Indusiate Sori: Covered by a protective flap called an indusium. In Dryopteris, the indusium is a single, cup-shaped structure (unilateral). In Polypodium, the indusium is absent, making them nude sori.
• Nude Sori: Lacking an indusium. In Polypodiaceae, sori are often round and arranged in rows along the veins.
• Compound Sori: In some ferns like Asplenium, the sori are elongated and may appear compound due to the fusion of individual sporangia.

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

The megasporophylls of Cycas species exhibit significant structural variation. In C. circinalis, the megasporophylls are broad, pinnate, and bear distinct, large ovules on the abaxial surface. The ovules are subtended by a bract-like structure. In C. revoluta, the megasporophylls are narrower, with more numerous ovules arranged in a dense cluster. The ovules are smaller and more compact. C. beddomei features compact megasporophylls with overlapping scales, providing protection to the developing ovules. C. rumphii displays intermediate forms, with megasporophylls that are moderately broad and ovules that are less densely packed than in C. revoluta.

**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. Ovule Structure: The ovule is essentially a megasporangium surrounded by an integument, resembling the structure in ferns.
3. Absence of Pollen Tube: Fertilization occurs via a pollen tube in some species, but in many Cycas species, the pollen grains release motile sperms directly, bypassing the need for a pollen tube.
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.

In conclusion, Cycas serves as a crucial link in the evolutionary history of seed plants, retaining ancestral traits while exhibiting adaptations for terrestrial life.