A herbaceous plant (in botanical use simply herb) is a plant that has leaves and stems that die down at the end of the growing season to the soil level. They have no persistent woody stem above ground. Herbaceous plants may be annuals, biennials or perennials.

Annual herbaceous plants die completely at the end of the growing season or when they have flowered and fruited, and they then grow again from seed.

Herbaceous perennial and biennial plants have stems that die at the end of the growing season, but parts of the plant survive under or close to the ground from season to season (for biennials, until the next growing season, when they flower and die). New growth develops from living tissues remaining on or under the ground, including roots, a caudex (a thickened portion of the stem at ground level) or various types of underground stems, such as bulbs, corms, stolons, rhizomes and tubers. Examples of herbaceous biennials include carrot, parsnip and common ragwort; herbaceous perennials include peony, hosta, mint, most ferns and most grasses. By contrast, non-herbaceous perennial plants are woody plants which have stems above ground that remain alive during the dormant season and grow shoots the next year from the above-ground parts – these include trees, shrubs and vines.

Some relatively fast-growing herbaceous plants (especially annuals) are pioneers, or early-successional species. Others form the main vegetation of many stable habitats, occurring for example in the ground layer of forests, or in naturally open habitats such as meadow, salt marsh or desert.

Some herbaceous plants can grow rather large, such as the Musagenus, to which thebanana belongs.

Plant pathology (also phytopathology) is the scientific study of plant diseases caused by pathogens (infectious diseases) and environmental conditions (physiological factors). Organisms that cause infectious disease include fungi, oomycetes, bacteria, viruses, viroids, virus-like organisms, phytoplasmas, protozoa, nematodes and parasitic plants. Not included are ectoparasites like insects, mites, vertebrate or other pests that affect plant health by consumption of plant tissues. Plant pathology also involves the study of pathogen identification, disease etiology, disease cycles, economic impact, plant disease epidemiology, plant disease resistance, how plant diseases affect humans and animals, pathosystem genetics, and management of plant diseases.

Plant pathogens

Fungi

The majority of phytopathogenic fungi belong to the Ascomycetes and the Basidiomycetes.

The fungi reproduce both sexually and asexually via the production of spores. These spores may be spread long distances by air or water, or they may be soil borne. Many soil borne spores, normally zoospores, are capable of living saprotrophically, carrying out the first part of their lifecycle in the soil.

Fungal diseases can be controlled through the use of fungicides in agriculture, however new races of fungi often evolve that are resistant to various fungicides.

thumb|left|[[Rice blast]], a necrotrophic fungus

Biotrophic fungal pathogens colonize living plant tissue and obtain nutrients from living host cells. Necrotrophic fungal pathogens infect and kill host tissue and extract nutrients from the dead host cells. See Powdery Mildew and Rice Blast images below.

Significant fungal plant pathogens include:

Ascomycetes

• Fusariumspp. (causal agents of Fusarium wilt disease)
• Thielaviopsisspp. (causal agents of: canker rot, black root rot, Thielaviopsis root rot)
• Verticilliumspp.
• Magnaporthe grisea(causal agent of blast of rice and gray leaf spot in turfgrasses)

Basidiomycetes

• Rhizoctoniaspp.
• Phakospora pachyrhizi(causal agent of soybean rust)
• Pucciniaspp. (causal agents of severe rusts of virtually all cereal grains and cultivated grasses)

Oomycetes

The oomycetes are not true fungi but are fungal-like organisms. They include some of the most destructive plant pathogens including the genusPhytophthora which includes the causal agents of potato late blight and sudden oak death.

Despite not being closely related to the fungi, the oomycetes have developed very similar infection strategies and so many plant pathologists group them with fungal pathogens.

Significant oomycete plant pathogens

• Pythiumspp.
• Phytophthoraspp.; including the causal agent of theGreat Irish Famine (1845–1849)

Bacteria

Most bacteria that are associated with plants are actually saprotrophic, and do no harm to the plant itself. However, a small number, around 100 species, are able to cause disease. Bacterial diseases are much more prevalent in sub-tropical and tropical regions of the world.

Most plant pathogenic bacteria are rod shaped (bacilli). In order to be able to colonize the plant they have specific pathogenicity factors. Five main types of bacterial pathogenicity factors are known:

1. Cell wall degrading enzymes– used to break down the plant cell wall in order to release the nutrients inside. Used by pathogens such as Erwiniato causesoft rot.

2. ToxinsThese can be non-host specific, and damage all plants, or host specific and only cause damage on a host plant.

3. Effector proteins These can be secreted into the extracellular environment or directly into the host cell, often via the Type three secretion system. Some effectors are known to suppress host defense processes.

4. Phytohormones– for example From Yahoo Answers