''Puccinia sorghi'', or common rust of maize, is a species of

rust fungus Rusts are plant diseases caused by pathogenic fungi of the order Pucciniales (previously known as Uredinales). An estimated 168 rust genera and approximately 7,000 species, more than half of which belong to the genus ''Puccinia'', are currently a ...

that infects corn and species from the plant genus ''

Oxalis ''Oxalis'' ( (American English) or (British English)) is a large genus of flowering plants in the wood-sorrel family Oxalidaceae, comprising over 550 species. The genus occurs throughout most of the world, except for the Polar region, polar are ...

''.

Host and symptoms

''Puccinia sorghi'' often first appears after silking in maize. The first early symptom includes

chlorotic In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...

specks on the leaf. The obvious sign of this plant pathogen is golden-brown pustules or bumps on the above-ground surface of the plant tissue. These bumps are

urediniospore Urediniospores (or uredospores) are thin-walled spores produced by the uredium, a stage in the life-cycle of rusts. Development ''Urediniospores'' develop in the uredium, generally on a leaf's under surface. Morphology *Urediniospores usually hav ...

s which can spread to other plants and cause further infection. They are circular and powdery, which result from

spore In biology, a spore is a unit of sexual or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plants, algae, f ...

s breaking through the leaf surface. While they are only about 1-2 mm each, they are very numerous with equal frequencies on upper and lower leaf surfaces. Over time, these blister-like bumps can change from brown to black, changing from urediniospores to

teliospore Teliospore (sometimes called teleutospore) is the thick-walled resting spore of some fungi ( rusts and smuts), from which the basidium arises. Development They develop in '' telia'' (sing. ''telium'' or ''teliosorus''). The telial host is the p ...

s. The most common place to find these spores is on the plant leaf, but they can develop on

husk Husk (or hull) in botany is the outer shell or coating of a seed. In the United States, the term husk often refers to the leafy outer covering of an ear of maize (corn) as it grows on the plant. Literally, a husk or hull includes the protective ...

tassel A tassel is a finishing feature in fabric and clothing decoration. It is a universal ornament that is seen in varying versions in many cultures around the globe. History and use In the Hebrew Bible, the Lord spoke to Moses instructing him to ...

s, and stalks as well. ''P. sorghi'' has two hosts making it a

heteroecious A heteroecious parasite is one that requires at least two hosts. The ''primary host'' is the host in which the parasite spends its adult life; the other is the ''secondary host''. Both hosts are required for the parasite to complete its life cycle ...

rust.

Maize Maize ( ; ''Zea mays'' subsp. ''mays'', from es, maíz after tnq, mahiz), also known as corn (North American and Australian English), is a cereal grain first domesticated by indigenous peoples in southern Mexico about 10,000 years ago. Th ...

and ''

'' are the two hosts for ''P. sorghi''. In comparison, the other common type of maize rust is southern corn rust ('' Puccinia polysora'') and it has a higher variety of hosts including maize, silver plumegrass, ''

eastern gamagrass ''Tripsacum dactyloides'', commonly called eastern gamagrass, or Fakahatchee grass, is a warm-season, sod-forming bunch grass. It is widespread in the Western Hemisphere, native from the eastern United States to northern South America.

'', ''

Tripsacum lanceolatum ''Tripsacum'' is a genus of plants in the grass family native to the Western Hemisphere. Gamagrass is a common name for plants in this genus. Species formerly included see '' Anthephora Apluda Chionachne Coelorachis Elionurus Hackeloch ...

'', '' T. laxum'', and '' T. pilorum''.

Disease cycle

There are five spore stages in ''P. sorghi''. The spore types are teliospores,

basidiospore A basidiospore is a reproductive spore produced by Basidiomycete fungi, a grouping that includes mushrooms, shelf fungi, rusts, and smuts. Basidiospores typically each contain one haploid nucleus that is the product of meiosis, and they are pro ...

pycniospore Pycniospores are a type of spore in fungi. They are produced in special cup-like structures called pycnia or pynidia. Almost all fungi reproduce asexually with the production of spores. Spores may be colorless, green, yellow, orange, red, brown o ...

aeciospore Aeciospores are one of several different types of spores formed by Rusts. They each have two nuclei and are typically seen in chain-like formations in the aecium. References Fungal morphology and anatomy {{mycology-stub ...

s, and urediniospores. Every year, viable urediniospores must travel to the north from the warmer southern climate. Since ''P. sorghi'' is an

obligate parasite An obligate parasite or holoparasite is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasite, ...

, it requires living plant tissue in order to survive. Therefore, this disease cannot overwinter in northern US states. The severity of the disease depends largely on weather conditions and how many spores are carried north each season. Urediniospores infect leaves and produce more spores to create a secondary inoculum and

polycyclic disease cycle Polycyclic may refer to: * Polycyclic compound, a cyclic compound with more than one hydrocarbon loop or ring structures, including: ** Polycyclic musks ** Polycyclic aromatic hydrocarbon *** Chlorinated polycyclic aromatic hydrocarbon *** Cont ...

. Once the urediniospores mature on the plant tissue and turn black they become teliospores. Urediniospores measure 22-33 × 20-28 µm. Teliospores are two-celled and measure 27-53 µm. Teliospores overwinter in the southern climate and germinate in the spring. Teliospores produce basidiospores which spread by wind to infect ''Oxalis''. They infect ''Oxalis'' and produce sexual spores (pycniospores) and aeciospores. Aeciospores are windblown to maize and infect the plant.

Management

The use of resistant maize hybrids is the best way to manage ''P. sorghi''. There are two types of resistance that exist. The first is partial resistance which results in fewer rust spots by reducing germination rate. This type of resistance makes ''P. sorghi'' less severe by slowing down development of number of urediniospores. The other type of resistance is qualitative. This type relies on a single gene which provides total resistance to the plant. Other management tactics include foliar application of

fungicide Fungicides are biocidal chemical compounds or biological organisms used to kill parasitic fungi or their spores. A fungistatic inhibits their growth. Fungi can cause serious damage in agriculture, resulting in critical losses of yield, quality, ...

and cultural control. For fungicide application, plants should be monitored throughout the season, spraying when there are six or more pustules per leaf. Fungicide groups that can be used include mixed modes of action,

DMI Triazole Propiconazole is a triazole fungicide, also known as a DMI, or demethylation inhibiting fungicide due to its binding with and inhibiting the 14-alpha demethylase enzyme from demethylating a precursor to ergosterol. Without this demethylation ...

s (Group 3), and

QoI Qo inhibitors (QoI), or quinone outside inhibitors, are a group of fungicides used in agriculture. Some of these fungicides are among the most popular in the world. QoI are chemical compounds which act at the quinol ''outer'' binding site of the ...

Strobilurin Strobilurins are a group of natural products and their synthetic analogs. A number of strobilurins are used in agriculture as fungicides. They are part of the larger group of QIs (Quinone outside Inhibitors), which act to inhibit the respiratory ch ...

s (Group 11). Cultural control can be more effective in areas where the spores can overwinter. Debris should be collected and destroyed by burning along with eradication of ''Oxalis'' in surrounding areas. In northern areas where the spores can’t overwinter, early planting time can help avoid ''P. sorghi''. Younger leaves are more susceptible to infection, by planting earlier the crop will be more mature and more resilient by the time the spores arrive.

References

{{Taxonbar, from=Q1665353 sorghi Fungi described in 1832 Fungal plant pathogens and diseases Maize diseases