Important Crop Diseases Caused by Fungi | Botany Optional for UPSC PDF Download

Diseases Caused by Fungi

• Fungi are responsible for the majority, around two-thirds, of infectious plant diseases. This category includes a wide range of diseases such as rusts (white and true), smuts, needle casts, leaf curls, mildew, sooty molds, anthracnoses, various types of spots (leaf, fruit, and flower), cankers, blights, scabs, and different forms of rots (root, stem, fruit, and wood). 
• Fungal diseases can also lead to wilting, as well as the development of galls on leaves, shoots, and buds. Virtually all economically important plants can be affected by one or more fungal diseases, often involving multiple fungal species causing diseases in the same plant species.

General Characteristics

• Fungi constitute a vast and diverse group of eukaryotic microorganisms. Their cells contain a nucleus enclosed by a membrane and lack chlorophyll, featuring rigid cell walls. Fungi often possess a plant-like vegetative structure consisting of microscopic branching filaments known as hyphae (singular hypha). Some hyphae extend into the air, while others penetrate the substrate they grow on, forming a network called a mycelium. 
• The mass of the mycelium gives fungi their characteristic "cottony" or "fuzzy" appearance. Fungi reproduce through various methods, both asexual and sexual, producing numerous types of spores in large quantities. For instance, the coloration of mold on bread is a result of countless microscopic mold spores.

Symptoms and Signs

• Fungal infections can lead to local or extensive tissue necrosis, hinder normal growth (hypotrophy), or stimulate abnormal growth (hypertrophy or hyperplasia) in specific areas or throughout an entire plant. Symptoms associated with necrosis encompass leaf spots, blight, scab, rots, damping-off, anthracnose, dieback, and canker. 
• Hyperplasia-related symptoms include clubroot, galls, warts, and leaf curls. In some cases, fungi infecting plants may produce growth or structures on plant parts like stems or leaves, such as masses of mycelium or spore aggregates with distinctive appearances. These are referred to as "signs" of infection, distinguishing them from "symptoms," which pertain specifically to the plant or plant tissue.

Transmission

• Fungi primarily spread via spores, which they produce in large quantities. These spores can be dispersed by wind currents, water (through splashing and rain), soil (in the form of dust), insects, birds, and the remains of previously infected plants. 
• Vegetative fungal cells present in deceased plant material can also be transmitted when they contact a susceptible host. The survival of vegetative cells from plant-pathogenic fungi in nature depends on environmental conditions, especially temperature and moisture. 
• Vegetative cells can endure temperatures ranging from -5 to 45 °C (23 to 113 °F), while fungal spores exhibit greater resistance. Spore germination, however, is favored by mild temperatures and high humidity.

Control

• Given the multitude of fungal species capable of infecting a wide range of plants, each with its unique characteristics, diverse control practices are employed to manage fungal diseases. 
• Key control measures include using disease-free seeds and propagating material, eliminating all plant materials that may harbor pathogenic fungi, implementing crop rotation, developing and utilizing resistant plant varieties, and employing chemical and biological fungicides.

Diseases Caused by Nematodes

• Nematodes that parasitize plants are slender, unsegmented roundworms, often referred to as nemas or eelworms. The majority of them are so tiny and translucent that they cannot be seen without the aid of a microscope. 
• Most adult nematodes fall within the size range of 0.25 to 2 millimeters in length. Approximately 1,200 species of nematodes are known to cause diseases in plants. It's likely that nearly every type of plant is susceptible to at least one nematode species. 
• These nematodes typically reside in soil and target small roots, but certain species can be found in and feed on bulbs, buds, stems, leaves, or flowers.

Mode of Nematode Attack

• Plant-parasitic nematodes obtain their nourishment by extracting plant juices. This feeding process involves the use of a hollow, needle-like mouthpart known as a spear or stylet. The nematode inserts the stylet into plant cells and injects a liquid containing enzymes that break down the plant cell contents. This liquefied material is then drawn back into the nematode's digestive tract through the stylet. Nematode feeding weakens the plant's natural defenses, reduces its vigor and yield, and makes it more susceptible to wilt-causing fungi, root-rot-producing bacteria, and other nematodes. Infected plants often exhibit signs of weakness that can be mistaken for drought stress, excessive soil moisture, sunburn, frost damage, mineral imbalances, insect damage to roots or stems, or disease.
• Typical symptoms of nematode damage include stunted growth, loss of green color (yellowing), dieback of twigs and shoots, gradual decline, wilting on hot, sunny days, and an unresponsive nature to watering and fertilization. Feeder root systems are usually reduced, appearing stubby or excessively branched, often discolored, and prone to decay. Winter damage to orchard trees, raspberries, strawberries, ornamental plants, and other perennials is often associated with nematode infestations.
• Root damage results from nematodes feeding on plant cells and the toxic salivary secretions of the parasite. Affected tissues may respond by enlarging or degenerating, sometimes both.
• While some nematodes are native and infest cultivated plants when their natural hosts are absent, others are introduced with seedlings, bulbs, tubers, and particularly in soil adhering to the roots of infested nursery stock.
• Nematodes may live partially free in soil around roots or in fallow gardens and fields. They can burrow into plant tissues (endoparasites) or feed externally on the surface (ectoparasites). They may enter plants through wounds or natural openings or penetrate through roots. All plant-parasitic nematodes require living plant tissues for reproduction. Nematodes are drawn to host roots by sensing the heat emitted by roots or the chemicals released by roots.
• Most nematode species complete a generation from egg to four larval stages to adult and back to egg in 20 to 60 days. Some nematodes have just one generation per year but still produce several hundred offspring.
• Soil populations and developmental rates of nematodes are influenced by factors such as the length of the growing season, temperature, water availability, soil nutrients, moisture levels, soil type, texture, and structure. Populations of nematode-parasitic bacteria, viruses, various nematode-trapping fungi, protozoans, mites, flatworms, or other pests, as well as other nematodes, also play a role. Toxic chemicals present in the soil or secreted by plant roots, crop rotations, past cropping history, plant species, variety, age, and nutritional status, among other factors, can influence nematode populations.
• Certain nematode species are strictly found in light, sandy soils, while others thrive in muck soils with high population densities. Light sandy soils are more prone to high populations and greater crop damage than heavy clay soils.
• Many plant-parasitic nematodes become inactive at temperatures between 5 and 15 °C (41 and 59 °F) and 30 and 40 °C (86 and 104 °F). The optimal temperature range for most nematodes is 20 to 30 °C (68 to 86 °F), although this can vary significantly depending on the species, development stage, activity, host growth, and other factors.
• Plant tissues can harbor large numbers of nematodes, with hundreds of thousands often present in infested roots or bulbs.
• Once a plant-parasitic nematode is accidentally introduced into a garden or field, it may take several years for the population to build up significantly (up to several billion or more active nematodes per hectare) and cause noticeable symptoms in a large number of plants. This slow progression is due to the sluggish movement of nematodes through soil, typically covering no more than 75 cm (29.5 inches) per year. However, nematodes can spread easily through the transfer of infested soil, plant material, or contaminated objects, including tools, machinery, bags, containers, water flow, wind, clothing, shoes, animals, birds, and infested planting stock.

Nematode Diseases

• One well-known group of nematode diseases is caused by root-knot nematodes, belonging to the Meloidogyne species. They are recognized for the distinct "knots" or gall-like swellings they induce on plant roots. Over 2,000 higher plant species are susceptible to their attack, leading to significant losses, especially in regions with extended growing seasons and warm climates. While some species, like the northern root-knot nematode (M. hapla), can be found in areas where the soil may freeze to depths of nearly a meter, common targets include vegetables, cotton, strawberries, and orchard trees. Garden plants and ornamentals are also at risk when infested nursery stock is involved.
• Root-lesion nematodes, from the Pratylenchus species, have a cosmopolitan distribution and are endoparasites. They cause severe damage to hundreds of different crop and ornamental plants by entering the roots and navigating through the tissues, breaking down cells as they feed. They deposit eggs that give rise to new colonies. Once a root starts to decline in vigor, nematodes venture into the soil in search of healthy roots. This leads to the formation of lesions in the root, with fungi and bacteria entering damaged tissues and causing root rot. Annual crops may succumb early in the season, but perennials and orchard trees may not show decline for several years.
• The golden nematode of potatoes, known as Heterodera rostochiensis, poses a significant threat to the European potato industry. Extensive efforts have been made to control it. The tiny golden cysts seen on infested plant roots are the remnants of female bodies, each containing up to 500 eggs that can hatch in the soil over a period of up to 17 years. A chemical released by potato and tomato roots triggers the hatching of these eggs.
• Another cyst-forming nematode species, the sugar beet nematode (H. schachtii), restricts sugar beet acreage in Europe, Asia, and the Americas.
• The citrus nematode (Tylenchulus semipenetrans) is found wherever citrus crops are grown and significantly affects fruit quality and production. Typical symptoms include gradual decline, yellowing and death of leaves, and dieback of twigs and branches in many groves that are 15 years or older. Infested nursery stock has played a substantial role in the widespread distribution of this nematode. The burrowing nematode (Radopholus similis) is a severe endoparasite in tropical and subtropical regions, where it attacks citrus (causing spreading decline), bananas, avocados, tomatoes, black pepper, abaca, and more than 200 crucial crops, trees, and ornamental plants, resulting in substantial losses.
• Numerous important ectoparasitic nematodes feed on plant roots, including dagger nematodes (Xiphinema), stubby-root nematodes (Trichodorus), spiral nematodes (Rotylenchus and Helicotylenchus), sting nematodes (Belonolaimus), and pin nematodes (Paratylenchus). Leaf or foliar nematodes (Aphelenchoides species) and bulb and stem nematodes (Ditylenchus dipsaci) cause significant harm to vegetable and ornamental bulb crops, clovers, alfalfa, strawberries, sweet potatoes, orchids, chrysanthemums, begonias, and ferns.

Control Measures

Controlling nematodes typically involves employing various strategies:

• Crop Rotation: Planting nonhost plants in rotation with susceptible ones can help reduce nematode populations in the soil.
• Resistant Varieties: Using plant varieties that are resistant to nematodes can be an effective approach in managing nematode infestations.
• Nematode-Free Nursery Stock: Ensure that nursery stock is certified as nematode-free to prevent introducing nematodes into new areas.
• Soil Fumigation (Nematicides): The use of soil fumigants, also known as nematicides, as preplanting or postplanting treatments can help control nematode populations.
• Heat Treatment: In confined areas like greenhouse benches and ground beds, soil can be subjected to steam or dry heat to eliminate nematodes. Exposure to moist heat, such as hot water at 50 °C (120 °F) for 30 minutes, is effective in killing most nematodes and their eggs. Higher temperatures may require shorter treatment times.
• Quarantines: State and federal quarantines may prohibit the movement of infested soil, plants or plant parts, machinery, and other potential carriers.
• Cultural Practices: Certain cultural practices can promote vigorous plant growth and nematode control. These practices include proper watering during droughts, appropriate fertilizer application, clean cultivation, fall and summer fallowing, using heavy organic mulches or cover crops, and plowing out roots of susceptible plants after harvest.
• Nematode-Repellent Plants: Some plants, like asparagus, marigolds (Tagetes species), and certain Crotalaria species, are known to be toxic to many plant-infecting nematodes and can be used strategically.

By employing these control measures and integrating them into agricultural practices, farmers can effectively manage nematode infestations and reduce crop losses.

Parasitic Seed Plants

Several flowering plants are classified as parasitic because they feed on other plants. Some of the notable parasitic plants include mistletoe, dodder, and witchweed.

Mistletoe

• Mistletoes are semiparasitic seed plants that grow on trees and obtain water and minerals by sending rootlike structures (haustoria) into the vascular tissue of the host tree's inner bark. There are three main types: American mistletoe, European mistletoe, and dwarf mistletoe. Birds often spread their sticky seeds. 
• Mistletoe can stunt or even kill branches on host trees. American mistletoe is well-known for its use in Christmas decorations. Dwarf mistletoe is particularly destructive to conifers in forests, leading to stunted, deformed, or dead seedlings and trees. Control involves removing infected branches before the fruits ripen.

Dodder

• Dodder, with over 100 species, is a leafless, threadlike plant that twines around various field and garden host plants. It lacks chlorophyll and depends entirely on its host for nutrients. Dodder can create tangled patches of infested plants. 
• It is often spread through contaminated field seeds. Control measures include planting certified, clean seed, mowing dodder patches before seed formation, burning dried patches, using selective herbicides, and planting resistant crops.

Witchweed

• Witchweed, belonging to the Striga species, is a small parasitic weed found in Asia, southern Africa, and the Sahel, and it has been introduced to other regions. Witchweeds parasitize the roots of many crops, including maize, sorghum, rice, and various grasses. They can severely stunt and weaken host plants, leading to reduced yields. 
• Witchweeds produce large numbers of tiny seeds that can remain viable in the soil for years. Control methods include using selective herbicides, crop rotation with resistant crops, prevention of seed production by growing and destroying trap crops, and keeping fields free of weed grasses that may host witchweed.

These parasitic plants pose challenges to agricultural and horticultural practices, and effective management requires a combination of control strategies.