By Dr. Thomas T. Yamashita

Many pathogenic diseases—diseases caused by viruses, bacteria, spores, or other organisms—are initiated through ‘passive transmission.’ That is, the infective unit finds its way to a host via wind, water, and other passive means.

However, many common diseases are transported actively through agents, which are referred to as ‘vectors.’ The formal definition of a vector is, “an organism or agent that transmits a pathogen.”

Mechanical vectors transmit disease-causing organisms, but don’t aid in their development.

If the vector merely serves as a passive agent of transport, and no development of the pathogen occurs on the agent, the vector is referred to as a ‘mechanical vector.’ For example, Dutch elm disease, an oftentimes lethal fungal disease of elm trees. The fungus is mechanically transmitted by the elm bark beetle (chiefly Hylurgopinus rufipes, but other species fall under the umbrella of the common name).

Biologic vectors not only transmit such organisms, but serve as hosts during reproductive phases.

An agent in or on which pathogen development occurs is a ‘biologic vector.’ A classic example of a biologic vector is the Anopheles genus of mosquitos, particularly Anopheles gambiae. Anopheles frequently serves as a host for the protozoan pathogen, Plasmodium falciparum.

P. falciparum multiplies and develops in the mosquito’s salivary glands, and is transmitted to hosts when the mosquito feeds. P. falciparum and four closely related species of protozoal parasites transmitted by Anopheles cause malaria. More than 200 million people are infected with this disease each year, with over 700,000 dying yearly.

Some vectors, known as paratenic hosts, are simply passive vehicles for their invisible infectious cargo.

At times, the agent is positioned in space and time to merely facilitate ease of transport or transmission of the pathogen. For instance, insectivorous rodents may acquire a nematode parasite that positions itself for the targeted end host: Cats which catch and consume the parasite-carrying rodents. This form of ecologically positioned agent is referred to as a ‘paratenic host.’ The act of passive transport and transmission by a paratenic host is referred to as ‘phoresis.’

For example, consider pine wilt disease, a serious disease affecting black and red pine trees. The causal agent of pine wilt is a nematode, Bursaphelenchus xylophilus, which hitches a ride under the wing casings of the Japanese pine sawyer beetle, Monochamus alternatus. When the beetle begins feeding on a new host pine tree, the nematode quits the beetle and enters the wounds created by the beetle’s feeding. The beetle is a paratenic host, and the nematode, B. xylophilus, is carried phoretically to the unsuspecting pine tree.

To understand the transmission of vectored plant diseases, it’s important to critically examine the identities and roles of their transmitting agents.

A host in which sexual reproduction of the parasite or pathogen occurs is generally referred to as the ‘definitive host.’ A host in which development is restricted to asexual or larval stages is called the ‘intermediate host.’

Overlapping these two types of host, a host which serves as a repository for pathogens, helping to keep sustain their population in a given area is referred to as a ‘reservoir host.’ Plants in particular fall prey to a vast number of diseases in which an agent is instrumental in transmission of the pathogen. By recognizing and controlling these agents, while improving the overall nutrition of affected plants, the effects of these diseases can be mitigated, or eliminated entirely

In the coming weeks, the Fusion 360 resource library will be updated with articles about several common vector-borne plant diseases. The articles will discuss their causal organisms, hosts, symptoms, and treatment options. The diseases we will focus on include:

• Pierce’s Disease of Grapevines
• Crown Gall of Walnut Trees
• Tobacco Mosaic Virus (TMV) of Tomatoes
• Prunus Necrotic Ringspot Virus (PNRSV) of Stone Fruit & Nut Trees
• Moldy Core (Core Rot) of Apples
• Pine Wilt Disease
• Ceratocystis Canker (Mallet Wound Canker) of Almonds
• Lettuce Big-Vein Disease
• Spring Dwarf and Summer Dwarf Disease (Crimp) of Strawberries
• Rhizomania of Sugar Beets

View these articles to learn more about the transmission and treatment of these diseases, or contact Fusion 360 today for personalized assistance with your agricultural needs.