A tobacco farmer is walking through her fields. On a tobacco plant leaf, she notices several small brown spots surrounded by what appears to be necrotic or dying tissue. What likely caused these spots and necrotic patches?

This is a fungal disease which is caused by  Alternaria alternata. This  is responsible for causing brown spot disease.

Explanation:

This is a most economically devastating disease which is caused by a pathogen called Alternaria alternata that mostly affects the mature leaves.

They survive in the debris and disperse through asexual spores.Their germination requires moisture and they attack the host plant through colonization and later propagate to another tissues.

They create irregular shaped dark brown spots which have yellow halos.

The pathogen tear away the narcotic tissues.

The spots get spread and creates curling leafs and moldy growth.

fertilization, pictured in figure 24.23a is the process in which gametes (an egg and sperm) fuse to form a zygote. the egg and sperm each contain one set of chromosomes. to ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. in mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. when a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. in placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in figure 24.23b. the fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. the nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

explanation:

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